You're asking about a topic that's been done to death, but there's a real difference between the iconic canon and the stuff that actually gets under your skin. Everyone's gonna mention 'The Things They Carried' and 'Matterhorn', and for good reason—they're masterpieces of the form. But O'Brien's book feels less like a 'Vietnam book' and more like a universal meditation on memory and truth, using the war as its canvas. 'Matterhorn' is just brutally immersive, a logistical nightmare novel as much as a combat one.
Where I'd steer someone new, though, is toward 'A Rumor of War' by Philip Caputo. It's nonfiction, but reads with the narrative force of a novel, and it's all first-person. It captures that slide from idealism into something much darker better than almost anything else. For a completely different, vital angle, 'The Sorrow of War' by Bao Ninh is the essential Northern Vietnamese perspective. It's fragmented, poetic, and utterly devastating, focusing on the aftermath and trauma in a way Western accounts often glance over. Le Ly Hayslip's 'When Heaven and Earth Changed Places' is another crucial one, giving voice to the civilian peasant experience in a way that complicates the whole conflict. Those last two stopped me cold and changed how I viewed the entire bookshelf on the subject.