These are a few books I recommend to students who want to gain a deeper understanding of Tai Chi & Qi Gong.

The Tai Chi Manual: A Step-by-step Guide... A good beginner's book which closely matches the syllabus that we study.  

Overcoming Arthritis Co-written by Paul Lam & Judith Horstmann, this book contains much information about coping with Arthritis and includes a photo-instruction set for Dr Lam's 'Tai Chi for Arthritis' set.  

The Theory and Practice of Taiji Qigong Excellent workbook for the Shi Ba Shi (the 18 movements of Taiji QiGong). This book is written for both beginners and more advanced practitioners, with explainations of the different focus and outcomes for both levels of practice.  

The Complete Book of Tai Chi Chuan: Another title from Master Wong, this book introduces the student to Tai Chi in its broadest context, examining the development of the many styles. There are line diagrams for several forms adequate for advanced students to learn from.  

Master Cheng's New Method of Tai Chi... Acclaimed translation of Professor Cheng's Treatise which introduced many of the concepts of Tai Chi to the West for the first time.  

A Complete Guide to Chi-gung Complementary to the study of Tai Chi is the study of Qi Gong, the nature of Qi itself and how it can be harnessed and developed.