Vietnam Above the Treetops
Can't remember much about it, but it's a classic.
What I recommend more than anything else ever is War Comes to Long An: Revolutionary Conflict in a Vietnamese Province by Jeffery Race. Race was the only civilian researcher in Vietnam and he interviewed government and military officials, VC defectors, any many others to cross-check events in building a picture of the ebb and flow of insurgency in Long An. VC makes progress, government devotes more resources, VC pull back under pressure, government withdraws resources to put them in high-threat areas, VC moves back in to fill the vacuum. He also explains the VC shadow or alternative government.
Character studies, background to steal, ideas on how to resolve higher-level efforts by the players, and more. Oh, and you'll come away with a vastly increased understanding of both the Vietnam War and the great difficulty of counterinsurgency warfare in general.