I highly recommend "Drawing on the Right Side of the Brain" I've never made it past Chapter 8, but the beginning chapters taught me to draw better than I had ever thought I could, while I was working on the exercises. The one stumbling block is the "Drawing Pane" (8x10ish piece of Plexiglass) that you really need to have in order to do the exercise properly, but maybe Pdaughter has one?

ETA: When I was doing Prop & Set Design at Lasalle College, my drawing teacher had us do several of the beginning exercises from "Drawing on the Right Side of the Brain" so it's got, y'know, actual academic cred and such.