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// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // etc..
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        // TODO Fill out the other cases

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}

// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // TODO etc.. fill the rest in
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        // TODO Fill out the other cases

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}

// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // etc..
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        // TODO Fill out the other cases

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}

Remember to always guard your inputs. We don't know if bookList is null or not and we are attempting to use it by calling a method on it. This will cause a NullReferenceException. Simply check if it is null before using it.

// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // etc..
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Make sure bookList is not null, since we will be operating on it.
    if (bookList == null)
    {
        // TODO Handle null bookList
        return;
    }

    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        // TODO Fill out the other cases

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}

The next issue with the code is the line this.mainMenu(b);. This causes a recursive call that can fill the stack if called many many times (via the input "1" -> "any key but 1" -> repeat).

// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // etc..
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}

Starting with a small nitpick, that the rest of the application code already has -- use full names for variables. Avoid the habit of short variable names like b. Feel free to use the full name of boardList instead. Remember that readability, clarity, and simplicity are key in programming.

The next issue with the code is the line this.mainMenu(b);. This causes a recursive call that can fill the stack if called many many times (via the input "1" -> "any key but 1" -> repeat) or worse, cause some interesting bugs later on.

Try adding another console output after this.mainMenu(b);, hit the above path a couple of times, and try to figure out what happens.

// Map user input (0, 1, 2...) to human-readable action names
// The actions should match up with the menuText string.
private enum MainMenuAction : int
{
    invalid = 0,
    repeat = 1,

    addBook = 1,
    deleteBook = 2,
    // etc..
    exit = 7,
}

// What we want our main menu to say.
// The numbers should match up with the MainMenuAction enum.
private static readonly string menuText =
    "Welcome to Library.\n" +
    "What do you want to do?\n" +
    "1. Add a book.\n" +
    "2. Delete a book.\n" +
    "TODO etc... fill it out here\n" +
    "7. Exit.";

public void mainMenu(BookList bookList)
{
    // Do our application-level looping here
    do
    {
        // Display the main menu text
        Console.WriteLine(menuText);

        // Get the user input
        int userChoice;
        if (tryGetInput(out userChoice))
        {
            // If user wants to exit, then exit the main menu.
            if (userChoice == (int)MainMenuAction.exit)
            {
                break;
            }

            // Handle the user input by casting the int to a human-readable name
            handleMainMenuAction((MainMenuAction)userChoice, bookList);
        }
        else
        {
            // TODO Handle unexpected input (parsing to int failed, user error)
        }
    } while (true);
}

private void handleMainMenuAction(MainMenuAction action, BookList bookList)
{
    // Ensure that the enum is defined
    if (!Enum.IsDefined(typeof(MainMenuAction), action))
    {
        // TODO Handle undefined enum (user error)
        return;
    }

    switch (action)
    {
        case MainMenuAction.addBook:

            // Command-level looping
            do
            {
                bookList.addBook();
            } while (askUserForRepeat("Add another book"));
            break;

        case MainMenuAction.deleteBook:
            bookList.deleteBook();
            break;

        // TODO Fill out the other cases

        case MainMenuAction.invalid:
        default:
            // TODO Handle invalid action (internal error -- not yet implmented, probably)
            break;
    }
}

// Just a wrapped version of int.TryParse that grabs from the command line
private bool tryGetInput(out int input)
{
    return int.TryParse(Console.ReadLine(), out input);
}

// Displays 'repeatText' as the first option, the second option is to go back to the main menu
// Basically a yes/no continue loop where 1 = continue and anything else = quit
private bool askUserForRepeat(string repeatText)
{
    Console.WriteLine("1. " + repeatText);
    Console.WriteLine("2. Back to Main Menu");

    int userInput;
    if (tryGetInput(out userInput))
    {
        if (userInput == (int)MainMenuAction.repeat)
        {
            return true;
        }
        else
        {
            // Go back to main menu -- user ended repeat.
            return false;
        }
    }
    else
    {
        // Go back to main menu -- invalid user input.
        return false;
    }
}