I've spent countless thousands of hours doing nothing but watching and learning body language thanks to working with and supervising boarding/daycare. A couple books I've read, "Other End..." for one, seem to be pretty spot on in what I've observed and learned myself. Body language is fascinating and each breed, and each dog, has it's own specific "accent" that not every other dog automatically knows how to read, so it's incredible to watch dogs learn how to "speak" another language.