As others have mentioned, the Volo's Guides are amazing. Lots of flavor and even if you don't play in the FR, can be mined for tons of ideas.

Many of the boxed sets for 1ed/2ed are amazing too. Some have been mentioned but I'll also add FR Ruins of Zhentil Keep and Greyhawk From the Ashes and City of Greyhawk. They don't make boxed sets like they used to. The poster maps alone are worth the price.

I hope you like eBay though, because that's what you'll need to use if want to hunt them down in print.