Books of the time were valuable. They were all hand produced, important and loved beyond the actual worth. So what do you do during a time when mythology and superstition ran things?
You include a written curse to protect the valuable thing you love!
Here are a few curses that were included in books of the time, that you can include in your own books (I recommend them being used in accurate replicas, but that are often not.
If anyone take away this book, let him die the death. Let him be fried in a pan. Let the falling sickness and fever seize him. Let him be broken on the wheel, and hanged. Amen.
The finished book before you lies. This humble scribe don’t criticize. Whoever takes away this book may he never on Christ look. Whoever to steal this volume durst may he be killed as one accursed. Whoever to steal this volume tries out with his eyes, out with his eyes!
This book belongs to none but me For there’s my name inside to see. To steal this book, if you should try, it’s by the throat you’ll hang high. And ravens then will gather ’bout to find your eyes and pull them out. And when you’re screaming “oh, oh, oh!” Remember, you deserved this woe.
This is the book of St. James of Wigmore. If anyone takes it away or maliciously destroys this notice in taking it away from the above-mentioned place, may he be tied by the chain of greater excommunication. Amen. So be it. So be it. So be it.
Whoever steals this Book of Prayer may he be ripped apart by swine, his heart be splintered, this I swear, and his body dragged along the Rhine. May no one believe that ever have I been taken, but that happily this place never have I forsaken. Yet may no one doubt that the wrath of God upon him will fall if he essays to take me from the confines of St. Gall.
The book of Saint Marie and Saint Liborius in Patherburnen. A curse upon the one who takes this book, a blessing upon the one who keeps it safe. If anyone removes or cuts a page, may he be accursed.
Whoever steals this book will hang on a gallows in Paris, and, if he isn’t hung, he’ll drown, and, if he doesn’t drown, he’ll roast, and, if he doesn’t roast, a worse end will befall him.
This book belongs to St Mary of Robertsbridge. Whosoever shall steal it, or sell it, or in any way alienate it from this House, or mutilate it, let him be anathema-marantha. Amen.
I John, Bishop of Exeter, know not where the aforesaid House is, nor did I steal this book, but acquired it in a lawful way.
Hanging will do for him who steals you.
There are many other curses of varying severest and entertainment. Marc Drogin compiled an interesting and entertaining book of curses, Anathema!: Medieval Scribes and the History of Book Curses.