The Morgan Horse, Jeanne Mellin. 1961. The original printing of this classic. Lots of photos. A good reference for the breeder & owner.
For the Good of the Horse, by Mary Wanless. An excellent guide to complementary therapies for the horse as well as topics such as dentistry, gait analysis, nutrition, saddle fit, and more.
The Nature of the Horse, by Margaret Cabell Self. 1974 by the classic author of horse books. Subtitle is "The origin, instinctive behavior, patterns, physical, mental and emotional attributes of the horse and how these affect the way we ride, train and handle him." A must read.
The Complete Book of the Quarter Horse, by Nelson Nye. Nye is better known for his western fiction, but does a good job of writing about the Quarter Horse. Written in 1964, there is much good history here.