Grimoires
Instructions for things that should not be attempted.
6 volumes
The most famous book that does not exist. A grimoire of pre-human history and summoning rites, surviving in a handful of guarded copies, quoted just often enough to convince generations of readers it was real.
A directory of the gods and demons of the Discworld, giving their names, addresses and summoning rites. Its author preferred to be known as Achmed the I Just Get These Headaches.
The spellbook the Creator left behind after making the Discworld, containing the eight Great Spells. It is chained in the cellars of Unseen University, and it has opinions.
The book that explains how to make a Horcrux, removed from the Hogwarts library by Dumbledore and kept in his office. Hermione steals it on the way out of the castle.
A book of dragon-summoning stolen from the Unseen University Library by a secret society, on the theory that a dragon would make an excellent political instrument.
A German study of surviving secret cults, printed in Düsseldorf in 1839. Its author was strangled six months later in a locked room, and the manuscript of his second book was torn to shreds.