Most of the books in the series are a bit absurd, or pretentious, or both, but I highly recomend Wendy Wasserstein's Sloth in Oxford University Press's Seven Deadly Sins series. Not only is it hilariously funny, it has a very real insight into sloth, and manages to poke fun in a clever way at our most peculiar form of sloth: self-help fads, which we use as a way to eliminate the need for that exceptionally good thing, hard thinking. It's a light, quick, enjoyable read with some serious thought behind it. Who could ask for anything more?
Wasserstein, for those who don't know (I didn't), is a major comic playwright.