Tai Chi for Dummies, by Therese Iknoian
Don't be put off by the title! This little gem, (written by a fellow exercise physiologist) is presented in a very down-to-earth and humourous style. The author approaches Tai Chi from a western viewpoint, and explains many of the underlying principles that can often seem rather abstract or arcane to our Western minds.
Highly recommended for both beginners and improvers.

Health for Life, by Bruce Kumar Frantzis
No forms or exercises to be learned in this one. Instead, an excellent book for all levels. For beginners an introduction, and for improvers a deeper understanding of Tai Chi and how it is applicable to our health, wellbeing and daily lives.

The Way of Energy, by Master Lam Kam Chuen
A best-seller for more than twenty years, this book is one of the very best about Zhang Zhuang (pronounced 'jam jong'). It's also the basis of a wonderful Channel 4 TV series:
http://www.youtube.com/user/StandStillBeFit
The book can also be downloaded in pdf format here:
http://www.eso-garden.com/specials/the_way_of_energy.pdf

The Tao of Health, Sex & Longevity, by Daniel Reid
Okay, we don't discuss the relevance of sex and Tai Chi in class, but if health and well-being is your priority, this fascinating book presents a truly holistic approach from a Taoist perspective. It goes where some of the other books don't, and shows how this ancient philosophical approach to life can complement our Tai Chi and Qigong.

The Tao of Pooh, by Benjamin Hoff
You've outgrown your Winnie-the-Pooh pyjamas, but this is definitely for grown-ups. A very easy to understand and fun introduction to the Taoist philosophy that forms the principles on which Tai Chi is founded. Not necessarily just for bears with very small brains.