I'm working on improving my drawing skills based on some free pdf-format textbooks. The books are guides to drawing by Andrew Loomis. He's amazing at drawing compelling images and he explains concepts in a very helpful and clear way. I was referred to him by the web site of an artist whose skill with facial expressions had impressed me.
They're so old that they're public domain. You can download them free for your own usage here: link to free books
I'm starting by flipping between "Fun with a Pencil" and "Drawing the Head and Hands". I've been frustrated for a while by my tendency to always draw the same sort of face, so I'm enjoying exploring new information. Loomis is very good at explaining how to bend the "rules" of face-drawing to make faces unique.