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];
    // ...
}

Both methods query Book__c by Id individually — slightly different field lists, but the same underlying "fetch this one book" shape repeated across the class.

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;
}

getBookById is private — an internal implementation detail, exactly like the helper methods extracted in Modules 12 and 17. Selecting a slightly wider field list once (covering every field any method needs) means every caller gets a consistent, complete Book__c without repeating the query's field list everywhere.

LibraryService library = new LibraryService();

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

Every existing caller of checkOutBook, returnBook, and the rest still behaves identically — 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.