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.

He called in Mr. Rasmussen, a professor at the University of Nevada in Reno and the author of “The Shakespeare First Folios: A Descriptive Catalogue,” who identified it within minutes.

“It was very emotional to realize we had a copy of one of the most famous books in the world,” Mr. Cordonnier said. “I was already imagining the reaction it would cause.”

Few scholars have yet seen the book. But its discovery among holdings inherited from a long-defunct Jesuit college is already being hailed as a potential source of fresh insight into everything from tiny textual variants to the question of Shakespeare’s connection to Catholic culture.

“It’s a little like archaeology,” James Shapiro, a Shakespeare expert at Columbia University, said. “Where we find a folio tells us a little bit more about who was reading Shakespeare, who was valuing him.”

The folio, whose discovery was first reported by the regional French newspaper La Voix du Nord, is not the rarest book the St.-Omer library owns. It also has a Gutenberg Bible, of which fewer than 50 are known to survive.

But few books hold the first folio’s value — one was soldat Christie’s in 2006 for $6.8 million — or its mystique. It contains 36 plays, nearly all of Shakespeare’s output. Printed in a run of about 800 copies in 1623, seven years after the playwright’s death, it is considered the only reliable text for half of his plays. (No manuscripts of any Shakespeare plays survive.)

Today, first folios are tracked like rare black rhinoceroses, right down to their disappearances. One is known to have burned in the Great Chicago Fire of 1871; another went down with the S.S. Arctic off Newfoundland in 1854.

New ones come to light every decade or so, Mr. Rasmussen said, most recently in the library of a London woman who died without a will. “It was a mess, with a bunch of second-folio bits mixed in,” Mr. Rasmussen said.

The St.-Omer folio, which is to be put on display there next year, will no doubt draw legions of visitors. It also, Mr. Rasmussen said, may feed one of the more contentious disputes in Shakespeare studies: whether the playwright was a secret Catholic.

That claim, Mr. Rasmussen said, has long been the subject of much “intelligent speculation,” most prominently of late by the Harvard scholar Stephen Greenblatt. The discovery of the folio in St.-Omer provides a bit more ballast, he said, if hardly a smoking gun.

Mr. Rasmussen pointed out the name “Neville,” inscribed on the folio’s first surviving page — a possible indication, he said, that the book was brought to St.-Omer in the 1650s by Edward Scarisbrick, a member of a prominent English Catholic family who went by that alias and attended the Jesuit college, founded when Catholics were banned from England’s universities.

“People have been making some vague arguments, but now for the first time we have a connection between the Jesuit college network and Shakespeare,” he said. “The links become a little more substantial when you have this paper trail.”

Jean-Christophe Mayer, a Shakespeare expert at the University of Montpelier, in France, cautioned against making too strong a connection, but noted that a library in the northern French town of Douai also owned some early transcripts of Shakespeare’s plays. “It’s interesting that the plays were on the syllabuses at these colleges,” he said. The new folio, he added, “could be part of the puzzle of Shakespeare’s place in Catholic culture.”

The St.-Omer folio will also help with the dizzyingly intricate piecing together of the most authentic versions of the plays. The text of each surviving first folio differs subtly from the others; compositors in the print shop constantly made corrections, introducing many textual uncertainties that still bedevil scholars and stage directors alike.

The St.-Omer folio, Mr. Rasmussen said, also contains handwritten notes that may illuminate how the plays were performed in Shakespeare’s time.

In one scene in “Henry IV,” the word “hostess” is changed to “host” and “wench” to “fellow” — possibly reflecting an early performance where a female character was turned into a male. “I’ve never seen this kind of gender switch in a Shakespeare folio,” Mr. Rasmussen said.

Even after years of inspecting first folios, Mr. Rasmussen sounded a little amazed at the discovery in St.-Omer.

“Here was a text everyone knew about, that had been in the library’s holdings for four centuries,” he said. “It’s about as ‘Antiques Roadshow’ as you can get.”