I really like "Wild Ride" by Ann Hagedorn Auerbach. It's about the rise and tragic fall of Calumet Farm, the nations premier racing dynasty, and is just a great story, as well as very informative. "Funny Cide" is also a great book.
Best of luck,
Claire Dorotik M.A., author, ON THE BACK OF A HORSE: Harnessing the Healing Power of the Human-Equine Bond"