Following is a list of books, a short description of them.  Many can be found at your local library, bookstore or tack store.
 

The Manual of Horsemanship: The official manual of the Pony Club this book is a great all around manual.   Written by the British Horse Society it includes riding, equitation, jumping, advanced training as well as horse care, veterinary notes, first aid among many other things.  The tenth edition is the first all around revision on the past editions.  Unfortunately sections such as lunging have been shortened extremely.  This is due to the British Horse society's publishing of the manual of "training the young horse".  However even with out the inclusion of such sections as a british to american terminology, updated medications and feeds as well as others.  The new edition is easier to read than previous editions.

The German Riding and Driving System Series:  A series of six books all translated to english.  Extremely educational in the training of the horse from the first stages up to higher levels of training.  The first two books the Principals of Riding and Advanced Techniques of Riding are indispensable for any english riders.  Created by the German National Equestrian Federation the series is based on the German training system which is derived from classical foundations.  Clear, logical and thorough the series is a great addition to any library.

Centered Riding by sally swift:  Sally swift a world renown trainer gives many visual aids in helping riders of any stage.  The book goes step by step through balance, equitation, the gaits and gives aids to help the rider develop a proper seat.  Ideas such as soft eyes, ice cream in your legs etc. are often what riders who have found themselves in a rut to get out of it.  A great aid for all riders english and western.

The Noble Horse by monique and hans d. dossenbach:
A large coffee table type book it traces the horse from prehistoric times to modern day.  Olympic disaplins as well as equine sports from around the world are included.  Breeds, and pictures of each, colors and genetics.  A good resource for the rider wanting to know more about horses, their behavior and their uses around the world.

Lyons on Horses by john lyons:  A great book for the horse owner working with young horses, breaking horses, or dealing with problem horses.  Round pen training, new ways to approach tying, loading, clipping among many other things.