I bought it because I had not a hope of interpreting other people's body language - it was a blind spot for me.
The book does what it claims to do. It explains why we use certain traits and how we do it, what to look for, what they mean. I found that it worked, I could observe people at work, social settings etc. and pick up things and interpret them - even found it quite fun. I use it in work too to my advantage. This week I explained what someone on TV was doing with their body language and even sounded like I knew what I was talking about - quite suprised myself!
A plus is that it explain WHY certain signals are made - which makes them easier to spot and interpret. Also it's written interestingly with a lively tone, helpful drawings, anecdotes and references to primates and primitive man to make it all fit together. Lots of photos of famous people and interpretations of their body language - very interesting!
It goes through body language methodically: hands, feet, face, arms, handshakes, head etc. which works well. I felt encouraged to use the knowledge in the real world. Lively, and encouraging, easy to read stuff.
A friend who could read the basics in people easily (as many people do) found this a bit of a case of stating the obvious. You may find this if you're quite an aware person, although I think almost everyone could gain something from this book.
If you're looking for a book because body language is not easy for you, there's no better starting point, I think. If you're looking for very advanced stuff, you may find you know all this already.