The book gives an excellent overview of Tai Chi's major styles, albeit a little more in the "Tai Chi solves everything" mode of those martial arts devotees that can veer towards cult-like obsession than I would like. It includes line-drawing illustrations of a form for each of Wudang, Chen, Yang, Wu, Wu, and Sun styles, which are largely excellent instructional tools for someone familiar with the art, but a little more explicit articulation of the differences in execution of some of the common moves would be helpful (e.g., Sun-style Brush Knee Twist Step v. Chen-style Brush Knee Twist Step). The rationale behind some of the forms is a little perplexing as well, since the author says that he's removed some duplication of techniques, but clearly left an abundance of others.
Overall, an excellent resource.