The military historian Nicholas Reynolds considers himself a lifelong fan of Ernest Hemingway. “I started reading Hemingway when I was in junior high,” he told me in a recent email exchange, and even at that precocious age he admired all the things you might expect a future military historian to admire in Hemingway’s work. “The characters he created embodied so many American values we still cherish,” Reynolds explains in the introduction to his new Hemingway biography, “Writer, Sailor, Soldier, Spy,” which enters the hardcover nonfiction list at No. 14. “Truth, bravery, independence, grace under pressure, standing up for the underdog.”

So it was only natural, when Reynolds was curating a 2010 exhibition at the C.I.A. Museum about the agency’s roots in World War II, to wonder if Hemingway, with his love of adventure, had ever spied for his country. “And then I learned something that surprised me,” Reynolds writes in the book: “He had signed on with another intelligence service, one that did not fit the conventional narrative of his life. That service turned out to be the Soviet N.K.V.D., the People’s Commissariat for Internal Affairs, the predecessor of the better-known K.G.B.”

Hemingway, a Soviet spy? “I was a traditional product of the Cold War,” Reynolds told me. “There was little sympathy for Communism in our house. So I felt like le Carré’s character George Smiley, who learns of yet another betrayal: I felt like I had taken an elbow deep in the gut.” But the more Reynolds dug, the murkier the picture became. Hemingway seems never to have provided any secrets to the Soviets, and was probably motivated more by anti-fascist politics than by any love for the Soviet Union or antipathy to America. The biography, then, is Reynolds’s attempt to tease out how his subject’s undercover life fit into the more familiar world of his work. “Hemingway did not write much about his spying, at least not for public consumption,” he told me. “His interest in intelligence emerges mostly from his letters, especially those to his close friend Gen. Buck Lanham, which include comments on Cold War espionage and the Red Scare of the late 1940s and early 1950s.” Hemingway’s voice, he added, “was uniquely American — and revolutionary only for its effect on the English language.”