* Fantastic Beasts and Where to Find Them: Originally presented as a textbook within the Harry Potter universe, it was later expanded into a full-fledged novel and the basis for a film series.
* Quidditch Through the Ages: Another fictional textbook featured in the Harry Potter series, it was later published as a real book.
* The Tales of Beedle the Bard: A collection of children's stories featured in Harry Potter and the Deathly Hallows, it was published as a real book with illustrations by J.K. Rowling.
There are also other books that are considered part of the Harry Potter universe, but are not technically spin-offs:
* Harry Potter and the Cursed Child: A two-part play that serves as a sequel to the original series.
* The History of Magic: A fictional textbook by Bathilda Bagshot, which was published as a real book to accompany the Harry Potter exhibition.
* The Case of the Christmas Pudding: A short story by J.K. Rowling that was published in a charity book.
So, depending on your definition of "spin-off," there are various books that can be considered part of the Harry Potter universe beyond the main series.