I highly recommend the following book on Tai Chi Chuan for beginner and intermediate students:
Tai Chi Chuan 24 & 48 Postures with martial applications by Master Liang, Shou-Yu and Wu, Wen-Ching.
For those who have learned the simplified 24, this is a very good reference book. It takes each posture and details the fighting or martial application. The book is highly recommned by several of the Tai Chi greats, who have provided the forward for the authors.
The cost was $16.95 at Barnes and Noble.