Politics, Trade, Banking, and Law: A book that would give structure and form for these areas of a campaign setting. I get the feeling most DMs just fall back on an eye-for-an-eye or fine PCs off the hip when laws are broken.
Planar Locations: These could be in book or adventure format. Just interesting places amongst the planes. In Sep's SH there was a fortress of blood floating in Limbo. When I read that I thought what a neat idea! I wish there was a book full of these kinds of things. You could stat up some denizens and a BBEG or two but leave the motivations completely nebulous. A DM could then drop this into an existing campaign VERY easily.
The Twilight Lands (ala Draconomicon): A high-quality resource on all things fey - not elven - and how to incorporate a more traditional fairey tale aspect into your game. Yes, I know about the articles on the WotC website; I would still want an actual book.
Survival Guide: The best chapters out of Frostburn, Stormwrack, and Sandstorm are the ones detailing the hazards of mother nature. I kept thinking to myself as I read them that a single book that addressed all of these different climates (without the proliferation of yet more feats, skills, and PrCs) would be a great resource for DMs.
Unerathe Arcana II: Because I just LOVE DM toolboxes. The first one was one of the best gaming books I've ever bought.
War: A book that not only covered mass combat but supply lines, but command structure, battlefields of various kinds (both mundane and magical), morale, fatigue, overland movement, food consumption, etc. At the same time, trying to keep things flowing as fast as possible.
