How to change the variable of an object specified in the widget argument

I'm a beginner and I cannot understand one thing. So, I have these classes:

class Book {
  int? id;
  Author author = Author();
}

class Author {
  String? firstName;
  String? lastName;
}

Creating the book object in the widget

class MainWidget extends StatefulWidget {
  const MainWidget({Key? key}) : super(key: key);

  @override
  State<MainWidget> createState() => _MainWidgetState();
}

class _MainWidgetState extends State<MainWidget> {
  final Book myBook = Book();

  @override
  Widget build(BuildContext context) {
    return ChangeBookAuthor(bookAuthor: myBook.author);
  }
}

And changing the value of the author

1. First example

   class ChangeBookAuthor extends StatefulWidget {
      final Author bookAuthor;
      const ChangeBookAuthor({Key? key, required this.bookAuthor})
          : super(key: key);
   
      @override
      State<ChangeBookAuthor> createState() => _ChangeBookAuthorState();
    }
   
    class _ChangeBookAuthorState extends State<ChangeBookAuthor> {
      late Author _bookAuthor;
   
      @override
      void initState() {
        _bookAuthor = widget.bookAuthor;
        super.initState();
      }
   
      @override
      Widget build(BuildContext context) {
        return TextFormField(
          onChanged: (value) => _bookAuthor.firstName = value,
        );
      }
    }
   

2. Second example

   class ChangeBookAuthor extends StatefulWidget {
      final Book book;
      const ChangeBookAuthor({Key? key, required this.book})
          : super(key: key);
   
      @override
      State<ChangeBookAuthor> createState() => _ChangeBookAuthorState();
    }
   
    class _ChangeBookAuthorState extends State<ChangeBookAuthor> {
      late Book _book;
   
      @override
      void initState() {
        _book = widget.book;
        super.initState();
      }
   
      @override
      Widget build(BuildContext context) {
        return TextFormField(
          onChanged: (value) => _book.author.firstName = value,
        );
      }
    }
   

In my application, I would like to have a ChangeBookAuthor widget that only gets the Author object and changes its value in the Book object in MainWidget. How can this be achieved? Why does the first example work and the second doesn't?