I’m half way through “The Artist’s Complete Guide to Drawing the Head” by Maughan. It teaches the chiaroscuro technique, that is rendering the drawing using light and dark shapes rather than lines. The nice thing I find about chiaroscuro is that it makes me focus more on the light and dark areas, which the key in making the drawing look three-dimensional.
The book suggests to use toned paper to act as the mid-tone, which I don’t have. This isn’t a deal-breaker though, I can tone the paper by lightly shading it. However it does makes it harder to create sharp highlights.