I would like to echo Fall's Street Without Joy.
I would also like to recommend:
Small Unit Action in Vietnam Summer 1966 by Captain Francis J. West, Jr., USMCR. The title speaks for itself.
Summons of the Trumpet, by Dave Richard Palmer. A very solid and interesting overview of the war, beginning to end.
Our Own Worst Enemy by Richard Lederer. Critical assessment of what was being done right, and wrong, from the early period when it could have made a difference.
The End of the Line: The Seige of Khe Sanh by Robert Pisor. Short book, but it covers the topic well. You can get some perspective on the importance of this series of battles by reading both Fall and Jules Roy's books on Dien Bien Phu.
Decent Interval by Frank Snepp. Insider account of the end of the war, and fall of S. Vietnam by a CIA analyst.
Platoon Leader by Frank McDonough. An excellent account of the development of a small unit leader. Consciously titled as a reference and homage to McDonald's Company Commander.
Baptism: A Vietnam Memoir by Larry Gwin. Gwin was a company XO in the Ia Drang period; his company was in the lead at LZ Albany. His facility with the language is unmatched in this type of narrative. He goes from introspection to bald facts without a hitch. There are many decent personal narratives from Vietnam. I think that this is the best.