by Scott Westerfeld (2006).
The totally awesome end to the Pretties trilogy, which I basically raced through. I love that conditions for our main character changed so radically from book to book; Westerfeld is certainly not afraid of mixing it up, or making drastic decisions in regards to his characters and worlds. I admire this immensely.
Ending a trilogy can be tricky, but I think this book pulled it off pretty well. The action was great, and nearly nonstop, and the end was a nice stopping place for a story that obviously goes on in its own world.
And, of course, there is the fourth book left to read.