The New Drawing on the Right Side of the Brain by Betty Edwards
I just recently saw this book at a friend's house and I remembered that Liam had recommended it a bunch of a years ago. And now I'm passing this recommendation on to you; everyone should have read this book before graduating high school.
Edwards starts off by breaking your brain; she theorizes that most people draw poorly because their drawing gets hijacked by the left side of the brain which wants to draw symbols of things instead of the actual lines. Her simple and effective solution is to turn the image of what you're drawing upside down. Don't think that your Left-Brain is holding you back? Try drawing something once right side up, and then turn it upside down and draw it upside down.
Edwards walks you through switching from Left-Brain use to Right-Brain; rendering edges and contours; negative space; perspective; portraiture (proportions, eyes, noses, ears); shading / cross-hatching; and color. As a final farewell, she includes a mini-tutorial on handwriting, which consists of a font review, and then using your newly-gained knowledge of contours, negative space, and proportion to improve your handwriting.