The City of Ember
For some reason, I've been through a spate of YA novels lately, and I think this was the best of them. Some very interesting ideas here, centered on what you would have to do when faced with the prospect of not having any natural resources to fall back on. The library full of books written by the citizens. The stores of reused, found, and repaired items (yarn taken from sweaters too worn to wear, for example). The way everything would slowly become shades of grey, with no bright colors left after centuries of handling.
In several other ways, though, this is as typical a kids' adventure story as humanly possible. Parents dead or otherwise out of the way, leaving room for young protagonists to act? Of course. Authority poking its nose in, unable to see the forest for the trees? Of course. Help from a trusted adult at exactly the right time? Of course. But that's just storytelling, and it works out fairly well in this case.
final thought: I dug it, but I haven't thought about it much since I finished it. I'd happily give it to a middle schooler, though.