In the authentication of documents, Mr. Coover said, most forgeries are readily apparent, typically because the paper cannot be faked. Such was the case with a supposed 1906 first edition of “Madame Butterfly,” purportedly signed and dedicated by the composer, Giacomo Puccini, which a reader of The Chicago Tribune asked Mr. Coover to authenticate.

Sight unseen, he was able to recite the dedication, in Italian (he said he had seen 10 to 15 copies of the score with the same words), and identified the reader’s find as only a photolithographic copy.

Then again, he said, ordinary-looking documents can contain surprises.

“An otherwise boring diary or series of family letters mainly recording weather and local news may contain a long description of an election campaign, demonstrations against the Stamp Act, the convening of the Confederacy to draft a constitution, or a raid by Pancho Villa,” he told the Williamsburg journal.

“The historical nuggets in original manuscripts are often buried, but rarely deeply,” he added. “I once discovered an exceptional letter of Ethan Allen at the bottom of a pile of old deeds, copies of minor poetry and otherwise uninteresting papers.”

Assessing the monetary value of an item is highly subjective, he said.

“Family Bibles and birth and death records are valuable for their genealogical information, but they have very little commercial value,” he was quoted as saying in Marsha Bemko’s book “Antiques Roadshow: Behind the Scenes” (2009), “and I think it is a shame to see little old ladies waiting in line for hours while hefting a 40-pound Bible that is worth very little monetarily.”

“You have to trust your innate instincts and perception of the size of the potential market,” he said. “The value of some letters and documents can only be determined by letting the free market operate, at auction.”

Mr. Coover recalled that in 1992 he was asked by the grandson of a woman who had recently died to appraise her collection of books. He visited her Manhattan apartment and immediately realized that the books were not very valuable, but as he was leaving, the grandson asked him to look at some papers in a tattered Manila envelope.

Inside, Mr. Coover told The Times in 2004, he found an old black leather book with the word “autograph” embossed in gold on the cover. On the very first page, he recognized Lincoln’s signature, followed by the last handwritten paragraph of his second Inaugural Address. He told the young man that that one page alone was worth at least $250,000. When it finally went to auction, it sold for $1.2 million.