First folios of Shakespeare’s plays are among the world’s rarest books, intensely scrutinized by scholars for what their sometimes-minute variations — each copy is different — reveal about the playwright’s intentions.

Now a previously unknown folio has surfaced at a small library in northern France, bringing the world’s known total of surviving first folios to 233.

“This is huge,” said Eric Rasmussen, an American Shakespeare expert who traveled to France over the weekend to authenticate the volume. “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.”

The book was discovered this fall by librarians at a public library in St.-Omer, near Calais, who were sifting through its collections for an exhibition on English-language literature. The title page and other introductory material were torn off, but Rémy Cordonnier, the director of the library’s medieval and early modern collection, suspected that the book — cataloged as an unexceptional old edition — might in fact be a first folio.