The Perfect Heresy, by Stephen O'Shea: I love this history of the Cathars. It tells who they were, how they fought, what they believed, and why the Church found them so insanely threatening that they proclaimed a Crusade against fellow Christians. Pretty much the perfect introduction to the subject, and loaded with adventure seeds for gamers. Highly recommended.
The Descent, by Jeff Long: This is an underdark novel set in the modern day, and written at a competent but hardly impressive level. There's a ludicrous romantic subplot, tons of gory death, and head-snappingly-abrupt changes of direction and style, but what the heck, right? It's a dumb romp in some ways and takes forever to get where it's going, but it's view of what an underdark society might look like, how it feeds and how it reacts to contact with the surface are all interesting.
White Stone Day, by John M. Gray: I thought his first book — the Fiend in Human — was amazing, and this is better. Great interwoven plots and points-of-view that make this Victorian thriller so much stronger than the average murder mystery. I'm swiping Robin and Weeks (two of his minor characters) for use as NPCs in my next game, and I'm generally just impressed with the skill and the deftness of his Victorian world. Excellent wry humor as well. A preview is available. Recommended.