Here is a list of readings that I find interesting and informative (please check with your local book/magazine dealers for current prices and availability):
Yoga: The Path to Holistic Health by B.K.S. Iyengar
Very informative book about poses/asanas, ailments, and philosophy. Also, has some sample class plans to help with certain ailments.
Hatha Yoga Illustrated by Martin Kirk
Book that describes poses, benefits, modifications and sample classes.
Anatomy and Asana – Preventing Yoga Injuries by Susi Hately-Aldous
Excellent book with anatomy pictures and descriptions.
Article about the benefits of yoga; Iyengar Yoga in this particular study.