Review and Refactor

public Checkout__c checkOutBook(Id bookId, String memberName) {
    Book__c book = [SELECT Id, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId];
    // ...
}

public List<Book__c> getBookDetails(Id bookId) {
    Book__c book = [SELECT Id, Title__c, Author__c, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId];
    // ...
}

1. public Checkout__c checkOutBook(Id bookId, String memberName) { — one method.
2. Book__c book = [SELECT Id, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId]; — queries Book__c by Id.
3. // ... — the rest of the method.
4. } — closes it.
5. (blank line)
6. public List<Book__c> getBookDetails(Id bookId) { — a different method.
7. Book__c book = [SELECT Id, Title__c, Author__c, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId]; — the same underlying "fetch this one book" shape, slightly different field lists, repeated across the class.
8. // ... — the rest of the method.
9. } — closes it.

private Book__c getBookById(Id bookId) {
    return [SELECT Id, Title__c, Author__c, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId];
}

public Checkout__c checkOutBook(Id bookId, String memberName) {
    Book__c book = getBookById(bookId);
    if (book.Is_Checked_Out__c) {
        throw new LibraryException('This book is already checked out.');
    }
    book.Is_Checked_Out__c = true;
    update book;

    Checkout__c checkout = new Checkout__c(Book__c = bookId, Member_Name__c = memberName, Checkout_Date__c = Date.today());
    insert checkout;
    return checkout;
}

1. private Book__c getBookById(Id bookId) { — private — an internal implementation detail, exactly like the helper methods extracted in Modules 12 and 17.
2. return [SELECT Id, Title__c, Author__c, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId]; — a slightly wider field list, covering every field any method needs.
3. } — closes the helper.
4. (blank line)
5. public Checkout__c checkOutBook(Id bookId, String memberName) { — now delegates instead of repeating the query.
6. Book__c book = getBookById(bookId); — every caller gets a consistent, complete Book__c without repeating the query's field list.
7. if (book.Is_Checked_Out__c) { — the existing guard, unchanged.
8. throw new LibraryException('This book is already checked out.'); — unchanged.
9. } — closes the if.
10. book.Is_Checked_Out__c = true; — unchanged.
11. update book; — unchanged.
12. (blank line)
13. Checkout__c checkout = new Checkout__c(Book__c = bookId, Member_Name__c = memberName, Checkout_Date__c = Date.today()); — unchanged.
14. insert checkout; — unchanged.
15. return checkout; — unchanged.
16. } — closes the method.

LibraryService library = new LibraryService();

Checkout__c checkout = library.checkOutBook(someBookId, 'Amara Nkosi'); // still works exactly as before

1. LibraryService library = new LibraryService(); — creates the service.
2. (blank line)
3. Checkout__c checkout = library.checkOutBook(someBookId, 'Amara Nkosi'); — still works exactly as before — this refactor only changed how the class fetches a Book__c internally, not what any of its public methods promise to callers.

This is the whole point of refactoring, revisited one final time to close out this project.