Why does Abraham Lincoln possess us so? Poet, philosopher, father, husband and president, he was a man who made himself remarkable in a remarkable time. He was, as Emily Dickinson called him, “the still Man,” who looms larger and larger as our most modern, melancholic and politically vibrant president. We have a multitude of books devoted to Lincoln — his language, his roots, his tumultuous marriage, his politics and his presidency — but Stephen Harri­gan, in his sixth novel, “A Friend of Mr. Lincoln,” offers us an acute and original portrait of Lincoln in the 1830s and 1840s, when our 16th president was still a young backwoods lawyer whose hair was “like a clump of crow feathers.”

But Lincoln isn’t the central concern of the novel. Harrigan’s hero is a fictitious poet, Cage Weatherby, “the Lord Byron of Illinois,” who becomes one of Lincoln’s trusted companions in Springfield, Ill. Cage rescues Lincoln from his depressive fits, helps him scrape off excess baggage from his speeches, saves him from a pointless duel and warns him against the “embrigglement” of marriage to Mary Todd.

He, though, is “caged” within his own psyche, “a zero at the base of his soul.” Cage wants to be “the poet of the unfinished,” to celebrate the raw beginnings of prairie towns, yet he cannot even finish his own tale. The poet’s father was a prosperous Massachusetts man, the owner of a pencil factory who believed in his son’s talents and sent him off to wander Europe for a year. But Cage’s father fell into ruin and killed himself, leaving his son adrift. Cage finally settled in Springfield, the owner of a tiny boardinghouse, and continued to scribble poems. He wasn’t tentative about the music of his lines. He realized that “music itself was not a human invention but a code buried in the suspirating rhythms of the world.”

He falls in love with a beautiful, young prostitute, Ellie, becomes her silent partner in a millinery shop and lets her live at the Palatine, his little hotel, where she can no longer entertain her special customers. Trying to entrap her, he traps himself. He is now her one and only “customer,” visiting her several times a week behind the doors of her shop, but she is still “ungraspable as a ghost.” We, as readers, recognize her dilemma. She did not want to become Cage’s chattel, or his secret bride.

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He’s much less of a fool when it comes to Lincoln and Mary Todd. Keenly aware of Mary’s sexual musk, he observes the silver necklace fastened to her throat: “It was as if she had put it there only to signal that it should be removed, that everything she wore was only for the purpose of making you understand that there was bare skin beneath it.” And Lincoln, the bumpkin lawyer, is ambivalent about landing in her web. “They say happiness can slow a man down,” he confides to Cage, “and I don’t have time for that.”

We grasp Lincoln’s largeness, his burning ambition to break out of his backwoods bearskin. “I’m like a character from Shakespeare,” he says. Yet he doesn’t envision himself as Hamlet, or Lear, or Prince Hal, but rather Richard III, the conniving, sweet-tongued, murderous king. And it’s to Harrigan’s credit that he reveals the calculating, almost evil side of Lincoln, who can talk about Cage’s talents, his ability to transcribe “the thoughts of angels,” and then allow him to drift in the wind, abandoning him out of some legal nicety. “If there’s honor in you,” Cage says, “I no longer see it.” He senses that Lincoln has found in Mary a perfect fit for his endless ambition. “The two of them . . . could be dangerous together.”

Still, the best part of the novel isn’t about Lincoln’s ambitions or Mary’s vengeful ways. The book finds its own proper music when Harrigan writes about Lincoln as a circuit rider. Twice a year, he joins a caravan of lawyers as it crosses the prairie to deal with legal matters in the small towns of Illinois, having to endure “red scabrous bumps of bedbug bites” as the circuit riders sleep three in a bed. “No one really slept . . . because they were all deliriously happy to be away from wives and offices,” eager to retell tales of court cases involving “castration by pocketknife” and similar horrors.

Even if Harrigan clips a bit off the wings of Lincoln’s better angels, he also offers us a powerful glimpse into what the great man liked to call his “real life.” In doing so, he provides us with a rumbling, rambunctious novel, full of its own raw life.

Jerome Charyn is the author, most recently, of “I Am Abraham: A Novel of Lincoln and the Civil War” and “Bitter Bronx: Thirteen Stories.” His new book, “A Loaded Gun: Emily Dickinson for the 21st Century,” will be published in March.

A Friend of Mr. Lincoln

By Stephen Harrigan

Knopf. 415 pp. $27.95