This fall, librarians at a public library in St. Omer, France who were readying an exhibition on British literature made an incredible discovery: A first folio of Shakespeare's plays. The books are among the rarest in the world; this latest folio brings the total to 233.

This Folio, which contains 36 of the Bard's 38 plays, was mislabeled as "an unexceptional old edition," according to the New York Times. It was printed in 1623, seven years after Shakespeare's death. Though the volume was missing its title page and other introductory pages, Rémy Cordonnier, director of the library’s medieval and early modern collection, thought it might be a first folio. He called in Eric Rasmussen, a professor at the University of Nevada and Shakespeare expert, to see if his hunch was right; Rasmussen was able to authenticate the book in just a few minutes.

"This is huge," Rasmussen told the Times. "First folios don’t turn up very often, and when they do, it’s usually a really chewed up, uninteresting copy. But this one is magnificent."

Each Folio is slightly different, and this one is no exception; it also features a few handwritten notes, including one in Henry IV where "hostess" Mistress Quickley was changed to "host" and "wench" changed to "fellow," which may indicate a gender switch when the play was performed.

It's possible that the Folio was left behind when St. Omers College—founded by Jesuit priests in 1593 to provide a Catholic education to English boys—was expelled from the area in 1762. (The college changed its name to Stonyhurst College and is now based in Clitheroe, England.) "This particular copy has the name Neville written on the first page," Rasmussen told NPR. "Neville was the alias that was taken by the Scarisbrick family, a family of Catholic nobles. And we know Edward Scarisbrick, who took the name of Neville, went to Saint-Omer College."