Historic discoveries don't happen every day at the auction houses, but Swann Galleries has a true find in its Nov. 22 autographs auction: a signed letter, dated Feb. 14, 1776, by Phillis Wheatley, a Boston slave and the author of the first book of poetry published by an African-American. Wheatley was held in high esteem during the abolitionist movement in the 19th century and is still revered, as shown by her inclusion in a new exhibition on slavery at the New-York Historical Society.

Until now, only 19 Wheatley letters were known, all in institutions in America and Britain. Scholars were unaware of this two-page letter she wrote to her friend Obour Tanner, a slave in Newport, R.I.

"This letter is an entirely new discovery," said Jeremy Markowitz, an Americana specialist at Swann, a New York auction house specializing in books, photographs, posters, prints and autographs. "It was not previously known to exist and has never been published."

Swann is confident it is genuine. "We not only had it authenticated by a Wheatley scholar, but also analyzed the handwriting, the paper and the ink," Mr. Markowitz said. "You can tell a period letter by the way the ink lies on the paper, the fluid style and the paper itself."

Swann will put the letter on view on Thursday at 104 East 25th Street and estimates it will sell for $80,000 to $120,000. (Christie's sold the last Wheatley manuscript that came up at auction - a four-page, 1773 poem titled "Ocean" - for $68,500 in 1998.)