Secrets of Xen'drik
D&D supplements are only going to appeal to a certain mindset - even avid players of the game are going to glaze over at pages of NPC and monster stats - and 'Secrets of Xen'drik' is not going to get any crossover appeal. However, for those of us who love the minutiae of constructed universes, this is a lot of fun.
Eberron's creater Keith Baker wrote this as a more in-depth look at the setting's adventure-continent. Xen'drik isn't hampered down with all the law and order, peasantry, and stability of the rest of the world. It's landscape is capable of shifts in terrain and weather, and there are no limits to the new territory adventurers can discover a days walk from the port of Stormreach.
I'm a sucker for the history as well. Like the setting as a whole, it is a fascinating upheaval of what is expected in a D&D template and doesn't leave out the best of game play. This is the supplement book to get if you're more interested in a straight-forward hack and slash dungeon crawl campaign, though there's plenty of room for subtlety.
