You won’t hear Nico Walker on a book tour anytime soon because he’s serving two more years in prison for bank robbery. But don’t wait to pick up his lacerating new novel about the horrors of war and addiction. “Cherry” is a miracle of literary serendipity, a triumph born of gore and suffering that reads like it’s been scratched out with a dirty needle across the tender skin of a man’s forearm.

The story of how this autobiographical novel evolved is almost as remarkable as the story of how its debut author survived. In 2005 and 2006, Walker served as an Army medic in Iraq, where he was commended for valor and saw many of his buddies blown to pieces. Returning to civilian life depressed and traumatized, he became addicted to heroin, a habit he funded with extravagant success by robbing 10 banks in four months.

In 2013, when Walker was behind bars in the Federal Correctional Institution in Ashland, Kentucky, his journey from hero to thief became the subject of a harrowing profile in BuzzFeed. One of many people struck by that story was Matthew Johnson, a publisher at the independent press Tyrant Books. Fascinated by the historical tradition of war vets taking up bank robbery, Johnson sent Walker books and encouraged him to write about his life. Eventually, through one of those wildly circuitous trajectories that make up the map of literary history, Walker’s disheveled manuscript ended up at Alfred A. Knopf, the nation’s most prestigious publishing house.

In a gracious and unusually detailed acknowledgment at the end of “Cherry,” Walker credits Tim O’Connell, his editor at Knopf, with transforming those typewritten pages into this tour de force. But when I contacted O’Connell, he claimed he did nothing but edit Walker’s manuscript as usual. “It is the fruit of his hard work and remarkable natural talents,” O’Connell said, “especially his voice, which is unlike any other. Nico simply poured everything he had into it.”

That sounds right – and true to the searing authenticity of this novel, which tries to answer the question, “How do you get to be a scumbag?” But in the process of laying out the road to perdition, Walker demonstrates the depths of his humanity and challenges us to bridge the distance that we imagine separates us from the damned.

We meet the unnamed narrator in 2003 when he’s a listless college student raised by a nice middle-class family. From the start, his tone is one of mournful candor with a trace of straight-faced wit. “I sold drugs but it wasn’t like I was bad or anything,” he says. “I wasn’t bothering anybody; I didn’t even eat meat. I had a job at the shoe store. Another mistake I made. No interest whatsoever in shoes. I was marked for failure.”

With the same rueful smirk, he enlists in the Army “because I’d been saying I would.” The inane tests, the screaming sergeants, the empty slogans – none of it impresses him. “You just had to remember it was all make-believe,” he says. “We were pretending to be soldiers. The Army was pretending to be the Army.”

But there’s nothing make-believe about the blood that’s soon gushing across these pages. As an Army medic, he goes on missions that are vaguely explained, often impromptu, frequently disastrous. His fellow soldiers are regularly called upon to brutalize the local people. The Iraqis, for their part, are experts at planting IEDs in the roads. “I was supposed to pretend to be some kind of great healer,” the narrator says, but his medical expertise rarely involves more than scraping up bits of his friends and zipping them in bags. “I was not a hero,” he says.

Of course, we’ve heard these stories before, in superb fiction and nonfiction by other soldiers. But Walker, 33, brings a raw and casual brutality to the narrative of battle. His rambling collection of chaotic anecdotes involve drugs and porn, acts of cruelty and kindness, unending boredom pierced by spikes of terror. These juxtapositions convey the fundamental disorder of the American mission and its deleterious effect on the young people forced to implement it. His language, relentlessly profane but never angry, simmers at the level of morose disappointment, something like Holden Caulfield Goes to War: “I’m glad I missed the battle because it was probably bulls – and the Army just murdered your dog anyway.”

But Walker also channels an even older novelist who saw the carnage of war. His prose echoes Ernest Hemingway’s cadences to powerful effect like this: “By the time it was fall you could tell we were all a little off. In that state none of us could have passed in polite society; those of us who’d been kicking in doors and tearing houses up and shooting people, we were psychotic. And we were ready for it to end. There was nothing interesting about it anymore. There was nothing.”

Ironically, that sense of sliding into the abyss accelerates when the narrator leaves the carnage of Iraq and returns to Ohio. Suicidally depressed, suffering flashbacks, blackouts and chronic insomnia, he grows so addicted that his entire life revolves around dope. “I was only ever afraid of one thing in my life,” he says, “that I wouldn’t be able to get heroin.”

Even as I hyperventilate about this novel, I’m wary of the tendency to romanticize criminals, to treat their descriptions of degradation as unconscious art, to feel aroused by the vicarious thrill of illegality. But I honestly don’t think I’ve been hypnotized just by this novel’s relentless horrors. No – it’s that unflappable voice. “Cherry” is written without an ounce of self-pity by an author allergic to the meretricious poetry of despair. In these propulsive pages, Walker draws us right into the mind of an ordinary young man beset by his own and his country’s demons. In the end, his only weapon against disintegration is his own devastating candor.