Leviathan, an alternative history in which Charles Darwin was a genetic engineer, and now the British are using fabricated beasts for war. On the other end of the spectrum are the Germans and their allies, known as the "Clankers", and they're using, as one might guess, massive machines to do their bidding.
The story centers on two people, namely Deryn Sharp, a girl masquerading as a young man in the Royal Air Force, serving aboard the Leviathan (a gigantic whale airship), and Alek Ferdinand, the son of the famous Archduke of Austria, who is in grave danger from his own country, seeing as a letter from the Pope names him the next emperor of Austria. He must avoid capture at all costs.
In Leviathan, the earlier book, Sharp and Ferdinand became allies under rather difficult circumstances, and in this sequel, they've reached Istanbul, and now Alek seeks to hide somewhere within the vast continent, seeking asylum through obscurity. He has little option, seeing as Austria and England are now at war, and among the Darwinists, he's essentially their prisoner, and will probably be shipped off to a jail as soon as the Leviathan lands.
Sharp, meanwhile, is working hard as ever to conceal her identity (poor Mr. Sharp!), and has run into the problem of what to do with Alek, seeing as he is responsible for helping save the ship, and certainly she doesn't want to betray a friend. At the same time she'd hate to be found guilty of treason, hanged and quartered.
I certainly enjoyed Westerfeld's unique alternative history. His characters are well thought out, the plot turned at all the right times, and it was beautifully illustrated by Keith Thompson. In all, if you enjoy Steampunk, historical fiction, political fiction, war novels, genetic engineering, and gigantic fighting machines, you'll probably love Behemoth as much as Leviathan.