I've always even at a young age been able to subconsciously read body language (to be fair this is true of most normal people). As I got older I studied it more extensively, from self-study to classes on... lets call it: "interrogation and persuasion".
As Turncoat mentioned Paul Ekman is one of the founding fathers on facial recognition infact he mapped out the entire repertoire of facial expressions and even came up with the concept of so called "micro-expressions". I highly recommend his training on micro-expressions and macro expressions if you have the time and money and aren't already adept at reading people. The systems he has created have even been used successfully by autistics to interact more naturally with others.
Beyond that, the concepts of cold reading and other forms of manipulation used by con-artists, "mentalists" and magicians is a great place to look for relevant information on the topic. Even the new concepts created by so called "Pick-up" artists has a whole slew of this information compounded together for the very purpose of getting women to sleep with you. Check out the book called "The Game" by Neil Strauss.
for a free, thorough and fairly accurate overview I suggest reading:
http://westsidetoastmasters.com/resources/book_of_body_language/toc.html