ON NOVEMBER 16, 1959, Steve Allen began his eponymous show at the piano. Wearing a gunmetal suit and dark glasses, he played a zippy right-hand riff before turning to face the camera. Snapping to the beat of an off-stage snare, Allen told a story that’s been repeated so often it’s become something like a fact. “A novel titled On the Road became a bestseller,” he said, “and its author, Jack Kerouac, became a celebrity. Partly because he’d written a powerful and successful book. Partly” — he paused to unspool another cadence — “because he seemed to be the embodiment of this new generation.”

Kerouac emerged from a shadow. Under the klieg lights, his sculptured face collected light like water, distributing it across his cheekbones and around the inkwells of his eyes. Clad in a smart gray suit, he took a seat by the piano. The Kerouac Allen’s audience (and the 350 million or so who’ve played the clip on YouTube) met was mesmerizing. Backed by Allen’s keyboard, he read from On the Road and Visions of Cody, bringing out the bebop of the words so expertly that one of the aforementioned netizens listened to the reading every night before bed for three weeks. That Kerouac claimed to have written On the Road in less than a month only added to his charisma; he seemed to be not just a poet but also a prophet, channeling exotic energy into his ecstatic prose.

Yet as Joyce Johnson explains in The Voice Is All: The Lonely Victory of Jack Kerouac, the most recent biography to treat the writer and by far the best, Allen’s guest that night was in many ways a myth. For one, he wasn’t nearly as American as his title “King of the Beats” implied. Born to French-Canadian parents in a Massachusetts mill town, Kerouac was an ethnic minority, a fact that’s been recognized in academia but infrequently reported to the public. And although Kerouac wrote the famous scroll of On the Road in three weeks, it took him years to arrive at the voice that animates the novel.

Tracing that voice’s development is the main project of Johnson’s book. Drawing on Kerouac’s early writing and unpublished material from the Kerouac archive at the New York Public Library, she follows his unexpectedly tortuous journey to unify the competing languages and cultures of his youth. In the season of Beat revival set off by today’s release of the film adaptation of On the Road, Johnson’s compassionate portrait of a deeply conflicted man has the potential to redefine Kerouac, a writer whom many of us think we know, but whose complexities have been mostly covered up by an alluring caricature.

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Jack, Johnson explains, was really Jean-Louis. His parents, Leo and Gabrielle, migrated from Quebec to New England with their families in the 1890s, trading farms for factory towns. Kerouac and his two siblings, Gerard and Caroline, grew up saturated in la survivance, the peculiar Franco-American attachment to family, faith, and joual, the dialect of French that was their native tongue. When Kerouac entered middle school in Lowell, Massachusetts, he’d already begun to write — in third grade he created “Kuku and Koko at the Earth’s Core,” a comic strip modeled after those at the corner store — but he rarely spoke in class. Drawn mostly from movies and street...