Quick read; aimed at 10-12 year olds, I'd say. Beautiful packaging, and amazing illustrations (can I give an extra star for the illustrations?)
Set at the outbreak of an alternate WWI, Westerfeld posits a Europe split between Clankers and Darwinists. The Clankers are technologically apt, having developed clunky, clanky war machines strongly resembling George Lucas' AT-ST walkers. And, in this reality, Charles Darwin discovered the secrets of DNA, enabling his followers to create biologically engineered creatures - including living airships.
Enter Our Heroes - a young girl of modest means who disguises herself as a boy to join the air force - and the heir to the Austro-Hungarian Empire, Prince Alexandr.
A fast-paced narrative, heavy on the action, ensues.
Honestly, it was a bit juvenile for me; I'd probably have liked it more if I were (much) younger. But I didn't dislike it either. And I really loved the illustrations: http://www.keiththompsonart.com/