Sometime in the coming year I may end up getting back to work on a project I never finished. Tentatively titled Vajra, it was my NaNo project for 2007; it fell short of the needed word count for the month and I ended up shelving it and turning attention to other things. But once it's finished and retitled, it'll be the third book I've written, more or less in a row, that deals with a kind of "Tokyo of mystery" or "enchanted Tokyo".
The first one you already know well: Summerworld. (And if you don't know about it, then by all means educate yourselves.) The second is currently being written: Tokyo Inferno. I hadn't been consciously trying to create a trilogy — past, present, future (sort of) — but that's what came out.
To that end, I'm thinking of bundling all three of them into one volume when they're done — or, at the very least, selling all three together as a single $30 set (perhaps as a convention-only special). The three are linked by many other things other than locale, so it only makes sense. This would be a long way off, of course, but it's something I find myself coming back to as a nice way to wrap up the package with a bow.