Poisonous books were found in the library at the University of Southern Denmark.
The covers of three rare tomes, which were published in the 16th and 17th centuries, had been coated in ''high'' amounts of deadly arsenic, an X-ray analysis of the books revealed.
It is likely that the books would have been coated in the substance to protect them against insects and other animals.
The books were being studied as their covers had medieval roots.
Speaking to Fox News, Kaare Lund Rasmussen, a professor at the University of Southern Denmark, said: ''The moment we put the X-ray beam on the green surface we saw the fantastic high amounts of arsenic.''