A mysterious gap in a dusty bookshelf gave the game away for a corrupt library worker who stole more than €800,000 (£541,000) of antique tomes from one of Germany’s most respected universities.

Now the trial of Reinhold K (he cannot be named before the hearing) , who slipped 16th-century botanical works under his long black coat, is set to expose the increasingly lucrative world of library theft.

Across Europe, thieves have been targeting ancient and well-stocked libraries, and then feeding their booty to unscrupulous booksellers and auction houses.

Last month, a librarian was found guilty of stealing a 16th-century edition of Chaucer’s works and more than 40 other volumes from Manchester Central Library. The Royal Library in Stockholm has been hit, while the Jagellonian University library in Crakow lost a 15th-century copy of a work by the astronomer Ptolemy, as well as books by Copernicus, Galileo and Kepler.

The University of Erlangen in southern Germany first became suspicious when it tried to locate the beautifully illustrated Herbal Book by Leohart Fuchs, written in 1543. It was regarded as a European treasure — and it was missing.

A closer search showed that some of the world’s standard classical works on botany, one written in 1768, another in 1762, had disappeared. But it took two years to unravel the scope of the theft, which seems to have stretched over two decades, and to catch the thief: the loyal and friendly night porter, Reinhold K.

“The thief didn’t have to break down a door or force a lock,” said Hans Otto Keunecke, director of the university library. “He had unrestricted access to our most priceless possessions. He worked for us for 40 years and had all the necessary keys.” Police found books stacked in his garage and followed the trail to a bookseller, Ludwig M, who allegedly commissioned the thefts according to the market value of botany and zoology classics.

The two men will stand trial next month.

Both have confessed but the prosecutor, Andreas Quentin, said that the defence team would argue that many of the books were stolen more than a decade ago. “Under German law, the statute of limitations can apply in cases of serious theft committed ten years previously,” said Mr Quentin. “We will have to determine precisely when all the many books went missing.”

The breakthrough for library thieves came about four years ago when an appropriate chemical mix was discovered that could wipe out library markings without leaving a trace.

As most library thefts are inside jobs, there is usually no need for chemistry: it is often enough to steal an ex libris seal documenting that the book has legitimately left the library stock. It appears that many of the booksellers approached by recent book thieves have not looked too closely at the volumes on offer.

The main problem appears to be budget cuts, which have slowed library stocktaking to a snail’s pace. One of the stolen books found in the thief’s garage bore the pencilled stock note: “Rev.22.1.62”. That shelf of books, in other words, had not been checked for more than four decades.

Vanishing acts

In 1996, two first editions of Newton’s Pricipia Mathematica and one of Galileo’s works were found missing from Cambridge University’s library

Sources: Royal Library Denmark, Cambridge University Library, Times archives

Times Recommends

• Rio Tinto four held as China prepares case
• Diplomats must keep heads down and trousers up
• When in Rome should one make like Berlusconi?