Thanks for the mention Michael…Did you get the hard copies I sent over airmail?
Not sure if these have been mentioned in this thread:
Design Sketching: http://www.designsketching.com/ (very good)
Drawing for Designers, Pipes, A
Sketching: drawing techniques for product designers
And for some ideas around the skills and knowledge associated with sketching and the use of other design tools, you could dip into something like Lawson’s What Designers Know…
James.