Leviathan is a YA novel set in the opening days of an alternate WWI.
Techno-Fantasy: 28 Darwin discovering DNA and creating hybrid creatures to serve the empire? The backbone of the German war machine being diesel powered mecha? Not that I'd believe that any of it were possible as envisioned, but I almost wish it were.
Neo-Victorianism: 25 As I said, I didn't for a second believe it was possible, but Westerfeld starting the book off with a young woman suspended above London beneath a lighter-than-air jellyfish? The book's dialog and descriptions made me feel like I was looking through the eyes of a lower class midshipman, and the scion of Austro-Hungary. It wasn't as all pervasive as Carriger's immersion, but still very well done.
RetroFuturism: 28 Without judgement (at least from the outside) Westerfeld paints a world of ubiqitious bio-genetic constructs and exoskeletal machines, set in 1914. A fanciful glimspe into both the past and the future. That's retrofuturism in a nutshell.
Other: 8 Wow.
Totally sucked me in. After Christopher Moore's Fluke I didn't think I would enjoy another book with pervasive bio-fabrications. However Darwinist creatures fascinate me. And then the Clankers, well their machines pull me back to my teen fascination with Aura Battler Dunbine. (there's not much overlap, but something feels like ABD) Of all of the current Steampunk authors, Westerfeld is probably the only book which grabbed me enough to want to buy the next book immediately.