Thinking about this three book thing, especially the adventure, I had a realization.
If the setting-specific adventure is a mega-module/adventure path that really draws on the unique flavor of the setting -- which is exactly what I'd expect if 1/3 of the lifetime physical material for a setting is an adventure -- there is a very good chance that I'd cycle through the settings pretty quickly (i.e. I'd up my book purchases).
The thing that kept me from ever really going whole-hog with Eberron was that there wasn't really a good AP for it. If I have to build the adventures, anyway, I may as well drop them in my home-brew. On the other hand, if WotC wants to send me on a guided tour of a setting, I'm right there.
Odds are good that not every setting will float my boat. I'll pass on FR, Spell Jammer, and Planescape, for sure. But, that just gives me sufficient time to explore Ravenloft, Dark Sun, and Birthright. Heck, knowing that there's a year's worth of adventure in the module and an escape clause at the end, I might even give Planescape a shot.
I'm actually really excited about that idea. So, door #1 is a 192 page adventure (that's RtToEE, IRRC). Door #2 is having a setting-of-the-year focus in Dungeon (I'm thinking an Adventure Path, still). Even better might be to open both doors up and give enough material in the adventure book to run a bare-bones campaign arc, but have the Dungeon adventures fill in gaps, offer alternate steps (i.e. the PCs find out about the tomb the lich wants to raid and beat him to it, rather than kill him), and side quests.
Squee....