Major John Plaster, a MACV-SOG veteran, wrote a memoir called 'Secret Warriors' as well and a history of SOG; both are great reading. So is Bernard Fall's 'Street Without Joy' about the French war in Vietnam. (And "Hell in a Very Small Place" about Dien Bien Phu.) 

Also David Hackworth's 'About Face': he is one of the most decorated soldiers of that war.