Slave Poet 's 1776 Letter , ` New Discovery , ' Is for Sale Historic discoveries do n'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 .