I read a book on body language about a year ago and it has been one of the most helpful books when it comes to telling if people are aggressive or non-agressive, telling a lie or the truth or telling if they are open or closed to comments being made in meetings. It is very entertaining to read the book and sit in a shopping center and just evaluate people walking by or in the food court and seeing people talking to other people. Just by looking at a persons hands, feet, eyes and mouth you can tell alot about what a person wants to do even if they don't say anything.
I know that LEO are taught alot of this as well as car salesmen. It has helped me alot and I plan on rereading it again just to try to absorb more of it.