Labyrinth by Kate Mosse
I have to admit that I started this one without the faintest idea what it was about (beyond "three secrets, two women, one grail," which couldn't be ignored on the front cover; I didn't read the back cover text). Obviously I knew something about it when I added it to my wishlist (right after it was first published), but since then I'd managed to forget whatever it is that I knew. Kate Mosse was one of the co-founders of the Orange Prize and, honestly, I'd give her a try just because of that.
Set both in modern day France and 13th century Languedoc, Labyrinth is a historical thriller revolving around the mystery of the Grail. What sets it apart from the other bestsellers with which it will be inevitably grouped is Mosse's understanding and evident love of the area in which the story is set (as well as her writing).
The one thing that was a bit annoying about the book was that (justifiably for plot reasons) Mosse holds out on explaining what the Cathar's truly believed until fairly late in the narrative. This probably wouldn't be a problem for readers familiar with the period, but for me I had a hard time understanding the justification for the crusade (which is the backdrop of the historical part of the novel) without knowing the extent of the heresy.
In any case, Labyrinth may be the best kind of historical fiction, the kind that makes you want to learn more about the period in which it was set. I'm actually kind of interested in reading Greg Mosse's Secrets of the Labyrinth despite the fact that I don't particularly care for spin-off books. Though I'll probably just see if Russell has anything on the period kicking around in his book collection.
Though this is really only tangentially related, I have to say that reading this book (in which the city of Carcassonne features prominently) made me want to break out Carcassonne, a fantastic euro board game...