An original copy of William Shakespeare's First Folio, one of the most sought-after books in the world, has been discovered “in plain sight” in the library of a 19th century Scottish mansion.

The book was published in 1623, seven years after Shakespeare's death, and contains 36 of his plays, including Macbeth, Julius Caesar and The Tempest. Eighteen of the plays in the First Folio would have never been recorded had it not been for the publication.

Emma Smith, a professor of Shakespeare studies at Oxford University, has verified that the nearly 400 –year-old book is in fact a genuine, original copy of the First Folio. But she admits that she was more than a little skeptical when she first received word of its existence.

"Pretty skeptical is putting it quite mildly actually," Smith told CTV's Canada AM on Friday. "This is one of the most studied and researched books in the world. I've been researching it for years and I thought I knew where all the copies are."

Smith said about 230 copies of the First Folio are known to exist. But it's been estimated that as many as 750 copies were produced.

The last copy was found in a library in Saint-Omer, France in 2012.

Smith said the "absolute proof" of the book's authenticity is in the paper's watermark.

"We know the paper stock that was being used in 1623 by the printers, the father and son team of William and Isaac Jaggard," Smith said. "So we know what watermarks are used and they're very, very difficult to forge."

Smith added that an inscription by Isaac Reed, an English Shakespearean editor who lived in the 18th century, also made her "immediately more interested" in the book.

"I knew that (Reed) had a copy of the First Folio, and we didn't know where it was."

The rare publication was actually sitting in the library at Mount Stuart House situated on the Isle of Bute -- a remote Scottish Island.

"An old book found in a library, how shocking is that?" Smith quipped.

"It was sort of there, in plain sight," she said. "It was one of those strange kinds of mysteries when something is there in plain view, but nobody quite knows what it is."

Smith said it's hard to pinpoint the value of the book, but she noted that in the past, original copies of the First Folio have sold for £3 million, or CAD$5.5 million.

"These first collected editions of Shakespeare plays are such charismatic books," she noted. "They're so important to literary history, and to the whole idea of English-speaking culture."