Some time ago I was interested to find extracts on Google Books from The Guild Handbook of Scientific Illustration by Elaine R. S. Hodges. Chapter 4 "Light on Form" was of particular interest. There are various resources online which give a partial explanation of the theory of light (at least in so far as it concerns artists) but this chapter is by far the clearest and most comprehensive explanation that I've yet come across. It may be of interest to other artists out there.
Aspiring artist, training in the classical tradition.