THE WORLDS OF DUNGEONS & DRAGONS - a hardcover with chapters giving overviews of every campaign setting ever published by TSR or Wizards of the Coast, with world maps and basic info. This would include Tekumel (maybe), Blackmoor, Greyhawk, Dragonlance, the Forgotten Realms, Planescape, Spelljammer, Birthright, Jakandor, Dark Sun, Eberron, Mystara, and a few others I'm undoubtedly missing. I'd of course love to see non-TSR/WotC settings included, but you've got to draw the line somewhere. The book would also include appendices detailing every product every made for each setting.