Cursed the ground where dead thoughts live new and oddly bodied, and evil the mind that is held by no head. Wisely did Ibn Schacabao say, that happy is the tomb where no wizard hath lain, and happy the town at night whose wizards are all ashes. For it is of old rumour that the soul of the devil-bought hastes not from his charnel clay, but fats and instructs the very worm that gnaws...HP Lovecraft, The Festival
The mind of a wizard is a restless, living thing. It is not the same mind as its body was born with, but is given to the wizard in life in exchange for his soul after death. It is always seeking to find and exploit secrets, and has a tenuous bond to its new owner. It is not at all uncommon, after a wizard's death, for his mind fail to die, and to seek new vessels. Many strange and unique cases of possession often follow the death of a wizard, and children born nine months after such a death always regarded with suspicion.
The mind of a wizard can be captured, however, and bound. A book, written by ritual, on vellum made from the wizard's skin, by ink made from their blood, will hold the mind, limiting its influence to those who read it. A stream of consciousness manifesto, autobiography, and spellbook, these tomes often find ways of being lost by their caretakers, then found by those more impressionable. Many wizards began their careers by finding one of these books.