'Balance in Movement' by Susanne Von Dietze. A great help if you're trying to improve your riding ability, regardless of which riding discipline you prefer. I'm making it required reading for all my serious students.

I have to second this title! It's one of the only books I've read on "balance in riding" that is really about proper biomechanical alignment of the body in all areas of life-- and that's what I find I need as I sit at a desk too many hours a day. Consciousness of proper alignment and "neutral position" all through the day is helping me find in on the horse so much more than just focusing on the time on the horse!