In a recent post, I mentioned an article from the British magazine New Scientist on non-verbals in the job interview situation (very analogous to the initial face-to-face sales call).
We do get into non-verbals particularly in my book Sales Presentations & Demonstrations.
But if you really want to know more, check out the website Simply Body Language. Lot of good stuff for the sales person. But also an array of funny videos and photos on when body language goes bad!