Traditional light and shadow drawing tutorials usually don’t cover one important aspect: you need to consider the purpose of what you are going to illustrate. For example, an illustration for the cover of a horror book will have a different lighting than a cover for a children’s book: in the first case, the lighting is usually more intense and dramatic.
This is what Blender Guru’s Lighting for Beginners video series is all about. Although it is aimed at Blender users, I believe it will also be useful for draftsmen and designers. Below is the intro video. To access the full playlist, click here.