Protect Your Library the Medieval Way, With Horrifying Book Curses

Or even more detailed: 

For him that stealeth, or borroweth and returneth not, this book from its owner, let it change into a serpent in his hand & rend him. Let him be struck with palsy & all his members blasted. Let him languish in pain crying aloud for mercy,  & let there be no surcease to his agony till he sing in dissolution. Let bookworms gnaw his entrails in token of the Worm that dieth not, & when at last he goeth to his final punishment, let the flames of Hell consume him for ever.

12th century Hell. Herrad von Landsberg/Public Domain.

via getpocket.com

That’s an idea.