Behemoth - Scott Westerfeld

The behemoth is the fiercest creature in the British Navy. It can swallow enemy battleships with one bite. The Darwinists will need it, especially now that they are at war with the Clanker powers.

On board the great airship Leviathan, Deryn, a girl posing as a hay, and Alek, the secret heir to an empire, are waiting to deliver a hidden cargo they hope will end the war once and for all. But their mission goes disastrously awry, and they find themselves hunted in hostile territory. As the lines between allies and enemies blur, Alek and Deryn will need brave hearts to face the consequences that follow…