It was traditional in medieval times when books were all hand written manuscripts to letter a curse into the book to prevent theft.
-Curse Against Book Stealers, Monastery of San Pedro, Barcelona
For him that stealeth a Book from this Library, let it change into a serpent in his hand and rend him. Let him be struck with Palsy, and all his Members blasted. Let him languish in Pain crying aloud for Mercy and let there be no sur-cease to his Agony till he sink in Dissolution. Let Bookworms gnaw his Entrails in token of the Worm that dieth not, and when at last he goeth to his final Punishment, let the flames of Hell consume him for ever and aye.
forestgrove via Libraryland