Books were so precious in the Middle Ages that monks invoked curses against any who might steal them:
This book belongs to S. Maximin at his monastery of Micy, which abbat Peter caused to be written, and with his own labour corrected and punctuated, and on Holy Thursday dedicated to God and S. Maximin on the altar of S. Stephen, with this imprecation that he who should take it away from thence by what device soever, with the intention of not restoring it, should incur damnation with the traitor Judas, with Annas, Caiaphas, and Pilate. Amen.
Should anyone by craft or 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 be recorded among the Blessed.
This book belongs to S. Alban. May whosoever steals it from him or destroys its title be anathema. Amen.
May whoever destroys this title, or by gift or sale or loan or exchange or theft or by any other device knowingly alienates this book from the aforesaid Christ Church, incur in this life the malediction of Jesus Christ and of the most glorious Virgin His Mother, and of Blessed Thomas, Martyr. Should however it please Christ, who is patron of Christ Church, may his soul be saved in the Day of Judgment.
These are from The Care of Books, by John Willis Clark, 1901. Happily, in 1212 a council met at Paris to decree that We forbid those who belong to a religious Order, to formulate any vow against lending their books to those who are in need of them; seeing that to lend is enumerated among the principal works of mercy.
From the present date no book is to be retained under pain of incurring a curse, and we declare all such curses to be of no effect.