Both of them are right

As Ashley says, both are right, but the meanings are different. "A page of the book" refers to a specific book, but not a specific page. "The page of a book" is more difficult. I would assume the identity of the page had been already suggested by context.

All right.But the meaning is different. The page of a book means the page you know or the only one of the page. A page of a book means the one of the pages