One night during his tour of duty as an officer in Vietnam, Tracy Kidder writes, “my men had gone to Chu Lai to see the new John Wayne movie about our war, “The Green Berets.” They had told me that the hundreds of GIs who were watching, in a grandstand outdoors, all started hooting and booing and hissing when the movie showed a scene of ocean and the sun setting over it. “This is what happens in Hollywood, of course, but in Vietnam, the sun rises over the ocean. It may seem a small mistake, but it is just the kind of falseness that forfeits the trust of the actual soldier or veteran. Only those who have been there, Mr. Kidder suggests, can be trusted to get it right.

This is true of any war, from World War II, now drowned in retrospective piety, to the current Iraq war, whose books and movies are just starting to appear. But it is especially true of Vietnam, which has carried such a heavy symbolic freight, at the time and ever since, that the unadorned truth about it is hard to come by. Maybe that helps to explain why Mr. Kidder’s writing life has been governed by a commitment to truth: His wide-ranging nonfiction books have tried to capture the human stories behind computer technology (“The Soul of a New Machine”), Third World epidemics (“Mountains Beyond Mountains”), and inner-city schools (“Among Schoolchildren”). Now, in “My Detachment,” his frank and funny new memoir of his Vietnam experience, Mr. Kidder undertakes something even more difficult: telling the truth about and against himself.

For what Mr. Kidder experienced in Vietnam was nothing to write home about. Literally: When he sat in his hootch in a small rear-echelon camp, isolated from Vietnamese civilians and NVA troops alike, Mr. Kidder had nothing interesting to tell, so he made up outlandishly self-glorifying stories. “I often wrote my parents about two Vietnamese boys named Go and Hanh,” he recalls, “and described various kindnesses I performed for them.” For his girlfriend Mary Anne, Mr. Kidder invented a different kind of exploit: “Speaking of integrity,” he quotes himself writing, “I rescued a pathetic little whore from the ocean today. God knows what she was doing there besides drowning.”

The 23-year-old lieutenant who wrote those boastful letters is separated from the author who mocks them by more than 35 years. That lapse of time is one of the explanations for Mr. Kidder’s canny title: He is sufficiently detached from the embarrassing young man he once was to write honestly about his Vietnam experience. In fact, the temptation for a soldier to embellish his war stories is one of Mr. Kidder’s major themes, and one of the main sources of his gentle comedy. Throughout the book, Mr. Kidder quotes passages from the unpublished novel he wrote just after returning from Vietnam – a callow Hemingwayesque affair chock-full of fraggings, atrocities, rapes, and all the other ordeals familiar from so many Vietnam books and films.

But while such events may have been, in a moral sense, the truth about Vietnam, they were far from the truth of Mr. Kidder’s own experience. Like the majority of American soldiers, Mr. Kidder never saw combat; he was what the front-line grunts referred to as an “REMF,” where “RE” stands for “rear echelon” and “MF” for a well-known 12-letter word. In charge of a field intelligence unit, his job was to receive bulletins about the location of enemy radio transmitters and plot them on a map, ostensibly to give his superiors advance warning of possible North Vietnamese attacks.

In fact, during Mr. Kidder’s year in the field, starting in June 1968, his position was never attacked, and he participated in only one major offensive, which was thwarted when the enemy escaped through underground tunnels. This left Mr. Kidder with no distractions from his second variety of “detachment” – that of an educated, middle-class soldier who found himself in a war he no longer supported. Mr. Kidder had gone from Long Island to Harvard, where he joined the ROTC as an alternative to the draft, but found himself ashamed to walk through campus wearing his uniform. After graduating, he was assigned to a post in Virginia and expected to sit out his enlistment stateside.

Getting transferred to Vietnam was a rude shock, especially for a young man who, as Mr. Kidder enjoys showing, had given little thought to the war, his military career, or life in general. Women, in particular, were a failed test. Some of the funniest and truest passages in the book have to do with Mr. Kidder’s outrageous manipulations of Mary Anne, whose attempts to break up with him were thwarted with emotional blackmail: “I had a face for occasions like this, an innocent, wounded boy’s face. I’d discovered it, not invented it, and I didn’t usually plan to assume it, but I could feel it coming on and was not unaware of its usefulness.”

Mr. Kidder proved more successful at passing another kind of test, that of military leadership. His small “detachment” of soldiers, the title’s last and most literal meaning, were as disaffected and disorderly as Vietnam could make them; the first words he heard from his enlisted men were “What the f– do you want?” As a recent college graduate without a shred of command experience, Mr. Kidder was hardly the officer to “clean up” the unit, as his captain blithely instructed him. Most of the book is devoted to Mr. Kidder’s comic blunders as a disciplinarian – frantically shining the men’s boots himself, for instance, when he panics at the prospect of an inspection by a higher-up. But in the end, he managed to grow into his rank, and even win the affection of his men. Certainly he wins the reader’s, with his humor and good nature.