John K, the creator of Ren and Stimpy, among other things, has an excellent animation blog where he frequently offers cartooning and animation lessons. The esteemed Mr. K can be dogmatic about what he views as the right way to do things, but a lot of the information he provides is really great. The latest one, which I found particularly interesting, is the fourth part of a discussion on animation backgrounds. From numbered page images, it looks like he is putting these together as a book, which I will be looking forward to.

Part 1: General Style Theory
Part 2: Contrasts in Shapes
Part 3: Contrasts in Direction and Size
Part 4: Organic Shapes and Forms
On simple color backgrounds