Should anyone by craft of any device whatever abstract this book from this place may his soul suffer, in retribution for what he has done, and may his name be erased from the book of the living and not recorded among the Blessed.
As book owners were so worried about theft and damage to their property they often included what is known as a “book curse” on the inside cover or on the last leaf of their manuscripts, warning away anyone who might do the book some harm. This curse is meant to frighten those foolish enough to belief in such things as curses, and yes, medievals tended to get scared off by this. Obviously it only works if you believe that the words printed in these “protected” books are special, potent magic, but as most people really did believe in it back than, the book curse was a most widely-employed and effective method of discouraging thievery. And to keep those at bay who didn't quite believe in book curses, books were often chained into place too.
For him that stealeth, or borroweth and returneth not, this book from its owner, 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 out for mercy, & let there be no surcease to his agony till he sing in dissolution. Let bookworms gnaw his entrails ... when at last he goeth to his final punishment, let the flames of Hell consume him forever.
I must confess, I really like the idea behind this. Though obviously it wouldn't work in our time. Then again, there are surly a lot of superstitious people still around, who'd rather not steal a book in a bookshop or library if there was a nice little book curse on the back of it. Or a chain that doesn't allow stealing in the first place.
By the way, while the first book curse above is a real one, the second one's a hoax written in the early 20th century.