From medievalbooks.nl: Chain, Chest, Curse: Combating Book Theft in Medieval Times.

Considering these two practical theft-prevention techniques – chaining your books to something unmovable or putting them into a safe – the third seems kind of odd: to write a curse against book thieves inside the book. Your typical curse (or anathema) simply stated that the thief would be cursed, like this one in a book from an unidentified Church of St Caecilia: “Whoever takes this book or steals it or in some evil way removes it from the Church of St Caecilia, may he be damned and cursed forever, unless he returns it or atones for his act” (source and image). Some of these book curses really rub it in: “If anyone should steal it, let him know that on the Day of Judgement the most sainted martyr himself will be the accuser against him before the face of our Lord Jesus Christ” (source). [continue]

(Link found here on the Schneier on Security blog.)