This is a fantastic book that's written by a former FBI agent and it's a great way to learn about body language. It shares invaluable tips and tricks and it's all fascinating.
You can put all the ideas in this book to immediate use and they'll really make a difference.
This isn't a subject that will just be of interest to professionals, as we all interact with people and we all send out so many non-verbal signals every day. It will help you work out how people really feel and stop you sending out the wrong signals.