The first in the Dungeons and Dragons Gazetteer. This is the thin (32 page) Greyhawk Gazetteer that came out during the early days of 3.0. I've been gaming for a while so I have quite a bit of Greyhawk knowledge, but still having handy summaries of the countries and gods is nice. Plus it's a nice supplement to the giant Greyhawk poster maps I have from Dragon Magazine.
The other book I picked up was Chill Vampires. I blame Tim at the The Other Side blog for this one. He's talked about Chill several times. I've never played it. The only other book I have for the system is Creature Feature. Still the book looked cool and I thought I'd pick it up.