We'll start this off with Problem Solving for Oil Painters: Recognizing What's Gone Wrong and How to Make It Right
Now, before all you watercolorists and colored pencil artists start yawning just hang on a second. One reason I want to talk about this book is because I thin the title is a little misleading. It really should be titled Problem Solving for Artists. There's very little in this book that doesn't apply to 2-D art of any medium. The artist/author, Gregg Kreutz, discusses things such shape, value, light, shadows, color, and depth, things important to consider whether you're using oil paints or pencils. He often shows comparison paintings of the same subject to clarify the differences he's talking about, which is very helpful.
Overall it's a great book for beginners and intermediates, although that doesn't mean some advanced artists won't find a few helpful points as well.