Rémy Cordonnier is in charge of a collection of over 50,000 ancient books kept in the provincial library of St-Omer, in the North of France.

He was preparing an exhibition on Anglo-Saxon literature when he found the valuable book of the Bard's plays.

"We knew we had this compilation of plays by Shakespeare, but it was wrongly catalogued as being a copy from the 18th century. When I put my hands on it, I saw clues which could indicate it was much older, such as errors in the paging," he told DW.

He got hold of the expert Eric Rasmussen, who authenticated it as one of the most valuable and coveted books in the world, a First Folio published in 1623. There were only 750 initial copies of this book, and each one is different. These errors made the book "immediately identifiable," the researcher told AFP.

Rasmussen spent two decades traveling the world to hunt down the remaining copies. His book, "The Shakespeare Thefts," tells of thrilling "run-ins with heavily tattooed criminal street gangs in Tokyo, bizarre visits with eccentric, reclusive billionaires, and intense battles of wills with secretive librarians," according to the publisher of his book, Palgrave Macmillan Trade.

The one found at St-Omer would be the 233rd to be unearthed, according to Rasmussen.

"What's most interesting about this copy is that the Henry IV play contains handwritten notes for a stage performance that might be from that period. Some female characters were changed into men," said Cordonnier.

Even though First Folio copies are valued between 2.5 and 5 million euros ($3.1 -6.2 million), the French library, whose invaluable collection also includes a Gutenberg Bible, will keep the book for research purposes.

The Shakespeare Folio will be on display in the exhibition planned for summer 2015.