COPENHAGEN (WP) - It sounds like the stuff of an Umberto Eco novel.

Three poisonous books were detected at the library of the University of Southern Denmark. Not, mind you, books about poison. The three texts are themselves poisonous, featuring covers made with high concentrations of arsenic.

The works are from the 16th and 17th centuries and cover a variety of topics.

Researchers uncovered the deadly tomes by accident. Initially, the professors X-rayed the books to try to determine whether any old Latin texts had been used in the binding.

"The library had previously discovered that mediaeval manuscript fragments, such as copies of Roman law and canonical law, were used to make their covers," Josh Povi Holck and Kaare Lund Rasmussan wrote in the Conversation.

"It is well documented that European bookbinders in the 16th and 17th centuries used to recycle older parchments."

But their scans revealed something else: The green paint covering the works contained the poison arsenic.

The researchers do not suspect nefarious ends. Arsenic-tinted green paint was common through the 19th century, even after people understood that the element was poisonous to ingest.

For a while, arsenic was considered safe enough to use as dye for stamps meant to be licked, and in dresses and ballgowns. It was not until the early 20th century that scientists understood arsenic could be dangerous when inhaled and that it could give off a poisonous gas.

The scientists theorise that the poison paint had been used to protect the books from insects and vermin.

For now, the books have been given a new home. Each is stored in a separate cardboard box marked with safety labels. The boxes will be stored in a ventilated cabinet.

Meanwhile, specialists will work to digitise each book so that researchers will not need to physically handle them.