Krepenevich's "The Army and Vietnam" is a good study, although it does focus more on the Army as opposed to politics. Palmer's "The 25 Year War" is also a good one, though it's older.

If you're looking for the feel of "on the ground," anything by Kieth Nolan is a must-have. There are also a number of good battle/campaign histories out there, starting with Moore's "We Were Soldiers...".