This is the first body-language book I've read that actually made sense to me, and which I feel I can trust. Rather than being written by TV personalities, with frequent appeals to speculative science, this one is written by someone who used body language for his day job in life and death situations over an entire career. He introduces a bit of (well-established) science, lots of empirically gathered experimental results, in case you're interested, but, mainly, he's talking about stuff he has observed over years and years, and personally put to the test.
Not surprisingly, this book makes far fewer claims for body language than some of the others I looked at. Navarro is categorical that body language alone cannot tell you a person is lying, although he does give some clear advice on what to look for. Rather, he focuses on barriers, pacifiers and emphasis which, when combined with the right questions, can lead you to seeing what areas a person is uncomfortable about. He quickly dismisses some of the grand urban myths of body language, for example that a person who touches their nose is lying, and makes some very good points from his criminal justice background about the dangers of believing such notions.
I learned a huge amount from this book, and it altered my thinking about what body language is and does. I learned even more about what it isn't and doesn't.
Chuck the other books away -- this is the one to buy.