It's possible you are thinking of:
* Fanfiction: Many fans have written their own stories based on the Harry Potter universe. These are not "fake" books, but rather creative works inspired by the original series. You can find them on websites like fanfiction.net.
* Unauthorized knock-offs: Some publishers have tried to capitalize on the popularity of Harry Potter by creating books with similar themes or titles, sometimes even using similar cover art. These are often considered low-quality and are not associated with J.K. Rowling.
If you can provide more context about the "fake Harry Potter books" you are interested in, I might be able to give you a more precise answer.