The Quiet American by Graham Graham (1955) is set partly in the Phat Diem area during the First Indochina War. The cathedral at Phat Diem features directly, and the novel's portrayal of the conflict between Vietnamese experience and Western intervention reads differently when you have stood in the building Greene describes. Still the essential novel about foreign involvement in Vietnam.
The Sorrow of War by Bao Ninh (1990) is the most celebrated Vietnamese novel about the American War — written from the North Vietnamese side, from the perspective of a soldier who survives the conflict but is destroyed by it. Raw, unsentimental, and required reading for anyone trying to understand the war's legacy in the country.
Catfish and Mandala by Andrew X. Pham (1999) is a Vietnamese-American's journey by bicycle through Vietnam, including the north, grappling with identity and the relationship between the diaspora and the homeland. Genuinely good travel writing, honest about the complexity of returning.