"The Dictionary of Body Language" is probably the only book you need to become an expert in deciphering human behavior, as far as that's possible, the only book you need to become a real-life Sherlock Holmes (by certain body signs not only can we tell about people's emotional states, but also about their occupations, as that could be achieved only by continuous observation).
Apart from the benefits of being able to read people properly, we also have the huge advantage of knowing the correct body language to display in front of others, so that we are perceived as we like.
One of the big pros of the book is that it dispels common myths about body language, most often related to recognizing lying, which would help us not to be deceived by the wrong information that's been presented to us so far.
Moreover, we had probably never even known about some of the body language signs presented in the book, considering that Joe Navarro paid attention to each part of the human body, including the neck, the hips, the buttocks and the genitals.
Furthermore, the different ways certain signs are perceived abroad and those that are unique for certain countries are covered in the book as well. So get ready to broaden your mind's horizons quite a bit when reading the book.
Lastly, I must mention something very important: if you aren't serious about learning the non-verbal cues we constantly give to other people, if you are the type of person who likes to read just for fun or to read self-help books without putting what you learn into practice, this book is certainly not for you for it will most likely bore you to death or you will totally waste your time reading it.