I'll give an example of a book I think fits this bill, the Neverwinter Campaign Setting. Its choc full of inspiring ideas.
Most good campaign setting books should be. Not that all campaign setting books are good, but still...
Most of my books are from the 3e era, and are fantasy in nature. I could put some of my original Traveler stuff up here too, but that's probably not really what you're looking for. I missed 2e entirely, and was disgruntled with 1e and BD&D.
Amongst campaign setting books that I think have tons of great ideas to mine, I'd put Five Fingers, The Pirate's Guide to Freeport, Sharn: City of Towers, the original Pathfinder campaign setting book, Heart of the Jungle, and Rule of Fear.
Other books that have given me that vicious GM gleam in my eye include the Fiendish Codex I, Drow of the Underdark, and The Manual of the Planes (3rd edition version, not the 1e version.) Green Ronin's The Book of Fiends also has a million and one great ideas, and Privateer Press's Monsternomicon books have each monster entry written up so that it's almost a mini-adventure in its own right, if not more.
My absolute favorite RPG book, though, and the one I turn to more than any other (yes, even for fantasy) is my d20 version of Call of Cthulhu. That's the most inspiring book, full of useable ideas, full of great concepts, and has the best advice for running the game that I've read almost anywhere.