Forgot to return that book?

For him that Stealeth a Book from this Library,
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 sink to 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 & aye.

From the Monastery of San Pedro, Barcelona, a blanket curse for the entire library...(I really wish this one existed, but unfortunately, it appears that it is apocryphal -- there is no monastery in San Pedro. It's so nasty though that I include it anyway.)

Always return books. As for stealing 'em...(shudder).