I found a great book a number of years back by the inimitable George Bridgman, called "
The Book of a Hundred Hands." If it's still around it'll help a lot, and it's in paper to boot, so it won't break the budget. Published by Dover, I believe.
Also, see Steven Sweeney's comments in
another post, on the drawing "Arlie in High-Key." It's another very thorough explanation/observation on high- vs. low- key drawing, and says it better than I.