Been a while since I read it, and it's not helicopter related, but I seem to remember really enjoying Pak Six by Gene Basel. I had read some of the other 105 pilot books, but remember this one being a stand-out. It seemed more 'written in the moment' somehow. I second the votes for Low Level Hell - brilliant book. Snake Pilot and Chickenhawk ditto.
On a side note, what a pleasure it has been to read some of the posts on this thread. I stumbled in looking for a few book recommendations, only to find that a few of the heroes I enjoy reading about are contributing. Very humbling indeed. I never lose sight of the fact that in learning to fly helicopters, I'm treading in the footsteps of giants.