The Body Language of Poker is an interesting book, and any serious poker player will certainly want to become familiar with its contents eventually. Its long (but well organized and nicely presented) laundry list of tells is certainly interesting reading, whether or not you think you can make adequate use of the information (I know I haven't been able to do so very often, but I haven't been playing very long). Some of the material seems like common sense in retrospect, but reading the book forces you to think about things you might have neglected otherwise. It certainly inspired me to pay more attention to the physical behavior of other players during hands I wasn't involved in. The text is lucid, and well-illustrated with photographs.
Caro also provides, among other things, estimates of the reliability of various tells in players of different skill levels, and of the value of picking up particular tells in different size games. It's probably wise to take these estimates with a grain of salt, but it's not reaching too far to claim that if you make intelligent use of the information inside, the book will pay for itself in short order (and earn you a substantial profit thereafter).
I was, however, very disappointed upon first reading the book to find that it doesn't really cover two areas that I would have thought would be crucial: eliminating your own tells, and picking up unusual tells in others. I know many poker players have systems and techniques for both of these, and it would have been nice to hear what Mike Caro could say about these topics.
Note that a new edition of this book came out in 2000.