Why does it keep giving me this error; "Book is not defined"

class Book:
    def __init__(self, author, title):
        self.author = author
        self.title = title

    def __str__(self):
        return f'{self.author}, {self.title}'

    def __eq__(self, other):
        if isinstance(other, Book):
            return self.author == other.author and self.title == other.title
        return False

from book import Book

class BookCase:
    def __init__(self):
        self.books = []

    def add_books(self, book):
        self.books.append(book)

# Example usage
personal = Book("John Green", "Paper Towns")
print(personal)  # Output: John Green, Paper Towns

bookcase = BookCase()
bookcase.add_books(personal)
print(bookcase.books)  # Output: [John Green, Paper Towns]