Second (or third?) World Builder's Guidebook. Had it for almost two decades now, and whenever I start up a new world, it's a resource I turn to. There's a bunch of tables to consult if you don't have an idea for what to put in an area. It usually leads to some weird stuff that, if you put a bit of thought and work into, becomes fascinating and interesting. We're talking stuff like high tech, mercantile gnolls with an arabic vibe, sky vikings with a fascination for settling near rivers, and theocratic halflings that subjugate dragons.
All of those are results that can easily happen if you just go full bore on the random tables... and to me, all of those are AWESOME. And if you don't like random tables, there's a bunch of other essays that give advice on building societies or terrain or pantheons. It covers the logistics of mapping a campaign, putting together interesting calendars or deities, and everything else.
I'd also recommend that you look AWAY from RPG products for the best advice on world building. Read some history, geography, anthropology, geology, or whatever it is that interests you. The more you read, the more ideas you'll get that can be dropped into your game. I know I've used everything from WW2 art's use of the "heroic scale" to an indepth knowledge of Cicero in my own games. My current campaign has drawn upon the Reformation and the rise of Protestantism (a current subplot), crossing the Roman emperor Hadrian with Joan of Arc, and weird weather systems that happen in archipelagos. Basically, I took as many real-world topics that interested me, twisted them, and put them in my campaign.
The other plus side about this is that you're broadening your interest in the real world. While RPGs are great and all, it's nice to be more well rounded as a person. My two cents, at least.
