From the Iyengar Institute:
Light on Yoga by BKS Iyengar
The classic guide to yoga. All yoga students should have this book. In addition to detailed instructions on over 200 asanas (postures), there is a comprehensive explanation of yoga philosophy.
Yoga in Action: Preliminary Course by Geeta Iyengar
Specifically geared towards students who are participating in Iyengar classes. Clear and easy-to-follow instructions on most of the postures encountered in the first couple of years of practice.
Tree of Yoga by BKY Iyengar
An accessible and easy-to-understand collection of talks and essays on yoga philosophy.
Yoga the Iyengar Way by Silva, Mira, and Shyam Mehta
For students from beginning to advanced levels. Includes an in-depth course of sequences to practice.
How to Use Yoga by Mira Mehta
Recommended for beginning students. Includes sequences with photographs for ease of understanding.
The Anusara website also has a long list of recommended yoga books for teachers.