Build the Core Classes

public class LibraryService {
    public Book__c addBook(String title, String author, String isbn) {
        Book__c book = new Book__c(
            Title__c = title,
            Author__c = author,
            ISBN__c = isbn,
            Is_Checked_Out__c = false
        );
        insert book;
        return book;
    }
}

1. public class LibraryService { — this is Module 14's Single Responsibility Principle in practice — one class focused entirely on library operations.
2. public Book__c addBook(String title, String author, String isbn) { — the entry point for adding a book.
3. Book__c book = new Book__c( — builds the record in memory.
4. Title__c = title, — sets the title.
5. Author__c = author, — sets the author.
6. ISBN__c = isbn, — sets the ISBN.
7. Is_Checked_Out__c = false — a new book starts available.
8. ); — closes the constructor call.
9. insert book; — the only insert book; this system should ever need, rather than scattered across every place in the org that needs to add a book.
10. return book; — hands back the now-saved record, with its Id populated.
11. } — closes the method.
12. } — closes the class.

LibraryService library = new LibraryService();

Book__c newBook = library.addBook('The Long Walk', 'Richard Bachman', '978-0-451-14098-6');
System.debug('Added: ' + newBook.Title__c + ' (Id: ' + newBook.Id + ')');

1. LibraryService library = new LibraryService(); — creates the service.
2. (blank line)
3. Book__c newBook = library.addBook('The Long Walk', 'Richard Bachman', '978-0-451-14098-6'); — any code that needs to add a book calls LibraryService.addBook() rather than writing its own insert statement.
4. System.debug('Added: ' + newBook.Title__c + ' (Id: ' + newBook.Id + ')'); — confirms it worked, keeping the actual DML logic in exactly one place.

Nothing stops any code from writing insert new Book__c(...) directly. The problem is the same one from Module 17's EmployeeDirectory: if adding a book later needs a validation rule (say, rejecting a blank ISBN), that rule has to be added to every single place that inserts a Book__c — unless there's exactly one method responsible for it. LibraryService.addBook() is that one place.