I wouldn't really bother reading much in the way of body language, I read a tome of a book called people watching and it contained everything.
Essentially motives are what make people convey messages through their body language, head down whilst walking= leave me alone, any part of a body being protected (legs crossed) conveys either a weakness or a need to protect something (legs crossed = protecting sexual organs).
In short skim read a book in a book shop then sit in a coffee shop for an afternoon people watching and rather than analyzing it let your subconscious do all the work for you, then reverse engineer the feelings (that guy looks creepy, why? etc).
As for mastering your body language there loads of resources scattered about the internet for PUA body language tips
