A book that is fantastic in the world it creates, giving information on the go and never in too large of bursts. As far as info-dumps are concerned the only overly done is in the beginning. From there it becomes a smooth ride of ups and downs, twists and turns. The world is amazing in the way it's constructed, everything is in chaos, run by gangs while the "better" beings are held separate and away. And yet, there is an amazing beauty to this after-world as well as a horror to the beauty of the paradise they go to. I won't say too much on the plot, since I'm bad at that, just know it is definitely a book to read.
As far as post-apocalyptic worlds go, this is the one that got me interested in such stories.