Three books that have had an impact on my practice in a notable positive way. In no particular order.
1. Way of Energy Lam Kam Cheun
For understanding the basics of your internal workings. A no nonsense, practical approach.
2. T'ai Chi Classics Waysun Liao
For an excellent modern translation of the Tai Chi Classics with a great deal of practical, step by step information.
3. A Study of Taijiquan Sun Lutang
Written by one of the greatest masters of the internal arts in the last century.