Want to learn more about horses? Pick a couple of these titles.

101 Arena Exercises	Cherry Hill	This one isn't meant for reading in front of the fire; this book is meant to be draped over the rail. Outstanding instructor's book with helpful exercises for all levels of horse and rider. Appropriate for all disciplines.
101 Longeing and Longlining Exercises	Cherry Hill Richard Klimesh	The companion book to 101 Arena Exercises, meant for schooling horses on the ground.
Anne Kursinski's Riding and Jumping Clinic	Anne Kursinski	An outstanding book for the huntseat rider from a USET member. A well written and well organized book which will actually help you become a better rider. Very highly recommended.
Basic Training of the Young Horse	Reiner Klimke	Simply a classic on the subject by a master.
Breaking and Training the Driving Horse	Doris L. Ganton	The guide to training driving horses. An absolute essential.
Centered Riding	Sally Swift	This will either be the most helpful book you've ever read, or the most useless book you've ever read. If creative imagery helps you learn physical skills, buy this book. If you think that sounds corny, leave it alone and buy Anne Kursinski or George Morris. It is a good read for instructors, regardless, since those who teach should study how others think and learn.
Common Sense Dressage	Sally O'Connor	One of the best books on dressage. Everyone will learn something, but relative beginners won't be left in the dust either. Highly recommended.
C.W. Anderson's Complete Book of Horses and Horsemanship	C.W. Anderson	This is just too much of a classic to leave out, despite the limited availability.
Dressage From All Angles	Barry Marshall	I really liked the organization of this book, which makes it ideal for both riders and fans of the sport. If you are having difficulity with a particular exercise, open to that chapter. If you are watching the Olympics and want more understanding of a particular movement, you can read the same chapter from the judges perspective. Probably my favorite dressage book.
Hunter Seat Equitation	George Morris	The classic guide to equitation and hunter style showing by the master. This book and Anne Kursinski's above make ideal companion volumes.
Lyons on Horses	John Lyons	Solid training and riding advice from a master of marketing. He wasn't the first to do what he does, but he brought these methods into the spotlight.
Practical Eventing	Sally O'Connor	Full of practical advice to those new to the sport of eventing, and quite a bit to help those who have been around a while as well.
Reining	Al Dunning	My personal favorite of all western performance books. I loved the practical advice, the very useful and well explained training methods, the personal stories, and the history. Al Dunning is one of my personal heroes.
Training and Showing the Cutting Horse	Lynn H. Campion	The best book available on the subject.