The book wasn't a commercial success," admits Tim O'Brien of his Going After Cacciato. His publisher peddled only 12,000 copies. But never mind. If John Cheever's Stories sold 115,000 and John Irving's The World According to Garp is the blockbuster (190,000 hardcover, more than a million paper), then O'Brien grabbed the glory—the National Book Award for last year's best fiction. "I needed it more than they did," jokes O'Brien, 32.

A powerful Vietnam novel, Cacciato was not without honor before its upset victory. Fragments, which began to appear in magazines in 1975, won two O. Henry awards, and upon the book's publication John Updike wrote in The New Yorker that O'Brien is "reaching toward a masterpiece."

He is also reaching back into personal experience. Tim was drafted after graduation from Minnesota's Macalester College in 1968 and sent to Vietnam. (His two prior books, If I Die in a Combat Zone and Northern Lights, also were influenced by his Nam experience.) "I was against the war, but going off to fight was tradition and obligation," he says. Once in combat he felt about as comfortable as Yossarian. When he arrived in boot camp, he notes, "I didn't know how to work a gun. I had a college education and figured they'd put me behind a typewriter. I guess I deceived myself." For eight months he was a foot soldier with the 198th Infantry Brigade. He received the Purple Heart after catching shrapnel near Mylai a year after the massacre, and eventually made sergeant.

To get through the daily horror and fear, O'Brien fantasized. "I would spend all my time imagining I wasn't there," he says. This is the stuff of the' picaresque Cacciato, and Paul Berlin, its antihero, escapes in his mind to Paris via Mandalay and Delhi. Is Berlin really O'Brien? "No, I am part of each character."

The author admires with reservations Vietnam movies like The Deer Hunter. "It's art, not real," he says. "I wanted a book that would make people feel the war." He began plotting his novel in 1973 while a graduate student in government at Harvard and by 1976 had decided to forgo the degree and complete the book, which went through 20 drafts. Tim and wife Ann, 30, a production manager at Sail magazine, still live in Cambridge, Mass. His distraction is golf—one endeavor in which he'd like to take on colleague Updike.

The son of an insurance salesman and a schoolteacher in "the turkey capital of the world," Worthington, Minn., O'Brien is "wiped out" by his sudden celebrity. "I'm not an ivory-tower writer," he shrugs. "I just wanted to write a good, kind of funny, kind of sad story. That's what I like to read."