I picked up a copy of James Richard's new book, Freehand Drawing and Discovery. And I have been testing out some of the principles in my drawings lately, when drawing crowds and how to incorporate people into a scene.
At first I was a bit leery, he gives some formulaic ways to draw people, trees, landscapes. But I see his reasoning now – a sort of "drawing training wheels" in order to you past the initial hurdles and out drawing. Once you are able to start producing your own work, then you can find you own style.
So for that, I did find this book interesting. He helps you find the structure in a complex scene and how you can render it quickly.