I recently picked up the Kobold Guide to World Building which includes essays by Keith Baker, Wolfgang Baur, David "Zeb" Cook, Jeff Grubb, Scott Hungerford, Chris Pramas, Janet Silverstein, Michael Stackpole, Steve Winter, and our own Jonathan Roberts (Torstan). These are essays rather than a woldbuilding toolkit, but it's all good stuff. I see that my development of the World of Kaidan, fits the premise of the essays, even though I never read it, prior to that creation.