by Scott Westerfeld (2005).
I am thoroughly addicted to this series. I'm liking it more and more as I read. This book might have felt very much the middle bit of a trilogy, but it was still awesomely fun. We learn more about the world outside the cities--that big wide world is hiding a lot--and about who are our lead characters are as people. The slang was used a little too repetitively, but all that annoyance got lost once the action scenes would kick up again.
I'm making myself wait for the third, but I'm very excited to get there.