Black April: The Fall of South Vietnam, 1973-75 by George J. Veith
It's a bit dry and tedious in spots, but since it is probably the first complete chronicle of the end of South Vietnam to use source material from both sides, I'm willing to give Veith a lot of slack. Especially since he has enough battlefield stories to make up for it. For example, he tells how the Battle of Ban Me Thuot turned on a broken firing pin... ("For want of a pin, the skirmish was lost. For want of a skirmish, a base was lost....")
Veith's main theme is that unlike what the American Lefties would have you believe, the government of South Vietnam was not a corruption-riddled cesspool, her military was not incompetent, and the people of South Vietnam most certainly did *not* want to welcome their Communist "liberators". And it was the shameful refusal of Congress to provide even minimal aid that basically signed their death warrant.