These are just a few books that are great to have on hand to give your practice a little more in-depth information and answer some of the "why" that may come up during class.  All of these make great companion references for both students and teachers alike.

The Key Poses of Yoga - Ray Long

Dr. Ray Long is a board certified orthopedic surgeon and a yogi with over twenty years of practice experience.  The book goes through virtually every pose of a standard practice with terrific illustrations of anatomy, as well as detailed explanations of how the body works.  It is perfect to just leave on your coffee table, pick up on your way to class, look at a specific pose, and then try to remember two or three of the specifics.  You will quickly notice improvements.

The Key Muscles of Yoga - Ray Long

Ditto from The Key Poses of Yoga, taking a deeper look into the anatomy of muscles and movement.

Science of Yoga: Understanding the Anatomy and Physiology to Perfect Your Practice - Ann Swanson

This book has a little bit of everything, in an excellently condensed format.  Cells, breathing, systems of the body, poses and their breakdowns, all accompanied with phenomenal illustrations.  Very easy to pick up and get drawn into and accessible to all levels of practitioners.

The Science of Yoga: The Risks and the Rewards - William J. Broad

This book examines some of the lesser known history of yoga as well as delving into some of the claims of its benefits and risks.  It is a very easy read that fairly addresses some misconceptions and truths, told with an appropriate amount of humor.  It ruffled some feathers when it came out, a portion of it questions who/what constitutes the 'authority' on yoga, but go into it with an open mind it and it is a fun and very informative read.