I think Satanism: The Feared Religion and What, the Devil? are some of the most important words on Satanism ever written. I find myself directing curious/ignorant people to the COS website to read the former, because it sums things up quite nicely. It would be nice if the latter was available online too for that same purpose, but oh well.
I also appreciate the quality of the book itself. The typography, graphics, and paper quality make it pleasant to look at. I also like the illustrations in the Rituals section.