Running an underground campaign

[Ahnehnois]

Always worth scanning the Far Corners of the World archives. DMGII has a few interesting bits on underground environments as well.

 

[Empirate]

I'd second (or third) Underdark as a useful book for this kind of campaign. Even the region descriptions hold lots of interesting hooks that can be refurnished to use in non-FR campaigns.

Dungeonscape I find one of the most disappointing books of all time. Except for Dungeoncrasher and Factotum, there's really almost nothing to hold my interest in there. Certainly very little in the way of mechanics, but unfortunately it doesn't even deliver on the fluff front IMO.