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.”

In “White Heat: The Friendship of Emily Dickinson & Thomas Wentworth Higginson” (Knopf, 416 pages, $27.95), the well-known biographer Brenda Wineapple provides a nuanced and quite moving account of the improbable relationship between an “activist” and a “recluse.” The story turns out to be far from straightforward. Not only is Dickinson the recluse almost impossibly bold, but Higginson the activist proves to be a man sharply divided against himself.

In that first letter Dickinson asked Higginson, “Are you too deeply occupied to say if my Verse is alive?” She wanted to know if her poems “breathed.” Dickinson didn’t want tips on prosody or corrections of her idiosyncratic grammar (though later editors would supply these lavishly). Nor was she angling for publication. It was the life in the poems that counted. The emphasis is significant. By asking Higginson to take the pulse of her work, she drew him steadily into the most delicate of entanglements. If he retreated, she advanced; when he pressed her, she withdrew. He later said, “The bee himself did not evade the schoolboy more than she evaded me.” But he too was a virtuoso of evasion. Their subtle pas de deux — an “intimacy sustained by distance,” as Ms. Wineapple describes it — is the real subject of “White Heat.”

Previous biographers of Dickinson have tended to belittle Higginson. He has been depicted as a well-meaning but clueless fuddy-duddy. As Ms. Wineapple brilliantly shows, that view is not only unjust but incorrect. From that first letter, Higginson had “the impression of a wholly new and original poetic genius.” And Dickinson herself, warmed by his constant encouragement, said more than once that he had “saved her life.” She addressed him as “Master” and “Preceptor,” and though her accents were teasing, her sentiments were sincere. His literary taste was conventional, but Dickinson wasn’t seeking literary approval so much as the gratifying electricity of a live response.

Still, Higginson was no mere sounding board. It is one of the conspicuous merits of Ms. Wineapple’s account that he not only holds his own with Dickinson (who, despite her claims to be “nobody,” overshadowed everyone around her), but comes to nervous life in all his bristling complexity. For Higginson was torn in his aspirations. He was both a man of action and a man of the word. He wrote and published profusely, but also advocated passionately as an abolitionist. He formed the first black regiment during the Civil War, a full year before the more celebrated Robert Gould Shaw formed his, and he not only led his troops into battle but sat down to dine with them at mess. And he campaigned tirelessly for women’s rights. He never became the poet he yearned to be; and perhaps it was the very purity of Dickinson’s aspiration, as well as the sheer strangeness of her genius, that drew him so strongly to her for so long.

Ms. Wineapple is especially astute on the flirtatious nature of the friendship. It wasn’t exactly a “dalliance,” but there was always the “faint hint of romance” hovering over the correspondence. Higginson’s wife, Mary, a lifelong invalid, disliked Dickinson; her disapproval may have made him keep his distance. (When Dickinson’s letters arrived, Mary would complain, “Oh why do the insane so cling to you?”) They met only twice, at the Dickinson home in Amherst, and after the first encounter, Higginson recalled thinking, “I am glad not to live near her.” That suggests that he was overwhelmed by her intensity; it could also imply that he was dangerously attracted. When Mary died and Higginson remarried — this time to a much younger woman — the letters perceptibly cooled. And when Dickinson died, on May 15, 1886, Higginson came to her funeral. He read one of her favourite poems by Emily Brontë, and he wept over her open coffin.

All his long life Higginson paid allegiance to large abstractions: Justice and Equality, Freedom and Beauty. For Dickinson, such abstractions were personal; like her poems, they were alive and breathing. “Circumference is my business,” she slyly wrote to Higginson, the busiest of men. He never understood that she shrank from the world to see it whole.

eormsby@nysun.com