By Scott Westerfeld. The other two books of the Uglies trilogy, so read Uglies first.
I will admit that I had trouble putting these books down. I read straight through them yesterday after posting about Uglies. They're certainly exciting enough, so what's the problem with them?
For one thing, God is completely missing. The closest to religion Tally comes is admitting, in a spooky forest, that she understood how people could start believing in spirits. This is an important omission, especially considering the subject matter of the book: what reason is there not to make pretties, specials and so on if humanity is not made by God, in God's image? Perhaps none... the villain of the series certainly didn't have any trouble with the idea.
The resolution of said villain's story also feels lacking. It's hard to say more without spoiling it, but the resolution lacks something.
Definitely an exciting trilogy, these books cover a lot of issues: environmentalism, beauty, human responsibility, brain damage... They make for a good read, but at the end, the story's resolution feels incomplete. Maybe that's why Westerfeld is working on a fourth book, Extras.