People Watching by Desmond Morris
This book comes from one of the most well known experts on body language in the world, Desmond Morris.
In this very comprehensive book, Morris accumulates some of the best research conducted in the area of body language and non-verbal communication. With lots of examples and extensive explanations, this book will provide you with an excellent advantage in understanding what people are saying without words.
I highly recommend this book particularly as a reference manual on the elements on non-verbal influence.