An excellent introduction to the art is Ultimate Guide to Tai Chi edited by John R. Little and Curtis F. Wong. Wong is the publisher of Inside Kung Fu magazine. The book has many reprints of article from the magazine, which are full of fascinating historical information about tai chi and its development. The book takes readers through the various styles and "families" of forms, including the Yang long form, which is the most popular form taught in America today. The book goes on to display a version of Yang long form in pictures — which is an invaluable tool for someone struggling to learn.
For those who already practice the forms, Lynda Myoki Lehrhaupt has gathered together thoughtful and often challenging essays on what tai chi means in Tai Chi as a Path of Wisdom. Not a how-to book as much as a jumping-off point for thinking about profound moments the art presents, Lehrhaupt's book never speaks down to the reader. In fact, in one essay, she discusses a master who compares standing with chi (or life force) to a little child with poop in her diapers. It led to hilarity in the class and a bit of shock for the author.
Both books ponder how this seemingly simple but deep art form brings peace and illumination to its devoted practitioners.