Ralph Waldo Emerson once wrote that each new acquaintance was potentially “an ambassador of the infinite.” For one of the Sage of Concord’s most fervent admirers, the now-forgotten reformer and man of letters Thomas Wentworth Higginson (1823-1911), the phrase proved surprisingly apt. On April 17, 1862, he received a mysterious unsigned letter in an almost illegible hand; into the envelope had been tucked four poems, along with yet another smaller, sealed envelope. Inside that he found a card on which the unknown sender had scribbled her name in pencil: Emily Dickinson. One of the poems began, “I’ll tell you how the Sun rose/A Ribbon at a time …” During the next 25 years, in letters at once riddling and coquettish — as well as in the more than 100 poems she would send the baffled but admiring Higginson — Dickinson too revealed herself “a ribbon at a time.”