Okay, I'm just going to say it. Leviathan is all sorts of awesomeness! The book takes place in an alternate reality at the beginning of World War I, which made me really glad I recenetly finished reading Paris 1919.
The two main characters represent each of the warring factions:
A 15 year old girl named Deryn Sharp dresses up as a boy to join the British Air Service. The Allied Powers in this universe are nicknamed the "Darwinist" after Charles Darwin who pefected a genetic splicing technique which allows the Allies to create their own new species. One of which is the Leviathan class air ship that Deryn is serving on.
The other is a young man Aleksandar Ferdinand who is the son of Arch-Duke Franz Ferdinand who's assissination starts what becomes World War I. The Central Powers are nick named the "Clankers" as they eschew the godless genetic experiment of the Allies and instead focus on steam powered machines. However, the Clankers are still influenced by the Darwinists as their machines tend to be modeled after animals. Such as the 8 legged walking dreadnaught which replaces the tanks we know of.
The universe that author Scott Westerfeld describes is fully developed and his characters act in believable ways. That is, not always in their best interest as they are kids after all.
In case you haven't guessed I give this book my must buy recommendation. Please check out the book's web site as well as it adds a number of extras as well.