"Some of the potions had effects almost too gruesome to think about, and there were some very unpleasant illustrations, which included a man who seemed to have been turned inside out and a witch sprouting several extra pairs of arms out of her head." (CS10)
Like anything else, books can be enchanted or even cursed with dire results for anyone who reads them.
The books kept in the Restricted Section of the Hogwarts Library are dangerous because they contain information about Dark Magic. These books are only read by older students who are studying advanced Defense Against the Dark Arts; a signed permission slip from a teacher is required to read any of those books. There are other dangerous books around too:
The Monster Book Of Monsters
Hagrid assigned this book to his first ever Care of Magical Creatures class. The book was nasty and vicious, trying to bite the hand of anyone who opened it. The only way to calm the book down was to stroke its spine, upon which the book opens placidly. Flourish and Blotts stocked the book for a while, keeping a cage of them in the front window, but they tended to tear each other to shreds and the owner swore he'd never stock them again. The only thing worse, he said, was The Invisible Book of Invisibility, which they never could find once they got them in (PA4).
Moste Potente Potions
This large and moldy-looking book includes the recipe for Polyjuice Potion. The pages are damp-spotted and the recipes are illustrated with grotesque pictures. "Some of the potions had effects almost too gruesome to think about, and there were some very unpleasant illustrations, which included a man who seemed to have been turned inside out and a witch sprouting several extra pairs of arms out of her head." Hermione kept this book out of the library for months while she, Ron, and Harry created the Polyjuice Potion in Moaning Myrtle's bathroom (CS10).
Sonnets of a Sorcerer
Of you read this book, you speak in limericks for the rest of your life (CS13)
Other unnamed cursed or dangerous books:
- a book that burns your eyes out (CS13)
- a book that you couldn't stop reading (CS13)
- a book that shrieks when you open it (PS12)
- books which describe how to create a Horcrux (DH6)
c.f. Tom Riddle's diary (CS13)
The recipe for the Polyjuice Potion can be found in Moste Potente Potions.