I loved this book; first because it's a thrilling adventure across Europe on the eve of World War I. However Westerfield has taken the mad science and technology of steampunk and inserted it into the timeline of these events. Here the Axis powers have developed mechanical technology along many of the typical Steampunk routes, creating large, walking machines of various sizes that can hold single spies or battalions. They also command fleets of zeppelins across a similar class range. The Allied powers have gone a biological route, genetically engineering creatures for domestic and military use. The titular Leviathan is the British Aerial Navy's enormous sky-whale that generates its own hydrogen to stay afloat. An ecosystem of other crafted animals (including birds, dogs, bats, and lizards) live in and around the beast providing it food and defense against attacks. The mad science aspect of the genre plays well with both factions, providing a balancing suspension of disbelief even when discussion of the technologies is given. 
 There are numerous illustrations within that add extra flavor to the descriptions Westerfield provides. I read this on a Kindle, but really feel I need to find the paper version to get their full effect. 
 The story follows two teenagers: Deryn Sharp from Scotland and newly with the navy, and Prince Alek of the Hapsburgs. Westerfield uses alternating viewpoints from these two to great effect in showing how each side views the other, revealing prejudices of the novel's world that stretch back decades before the beginning of the war. The characters also strongly parallel each other both in having lost family and hiding crucial secrets about their identities. This leads to a natural comradery when they do meet later in the book that transcends their opposing factions and genders. This is facilitated in the story by the fact that Deryn is masquerading as a boy but she plays up that part consistently even in internal monologues. There's a softer side to her at times, but she really is a tom-boy who would much rather be climbing the side of a living airship thousands of feet in the air than wearing dresses and being proper. Alek is a prince, and carries himself with a lot of the airs stereotypical to that character type. However events in the book scuff some of that polish and reveal a strong, caring person underneath, ready to do what he feels is right regardless of consequence. The supporting cast is also great, carried by some very witty dialogue, which I think Westerfield may be a master of.