Leviathan

Choose Your Weapon: Beastie or Clanker

Alek is a prince without a throne. On the run from his own people, he has only a fighting machine and a small band of men.

Deryn is a girl disguised as a guy in the British Air Service. She must fight for her cause--and protect her secret--at all costs.

Alek and Deryn are thrown together aboard the mighty airship Levithan. Though fighting side by side, their worlds are far apart. British fabricated beasts versus German steam-powered war machines. They are enemies with everything to lose, yet somehow destined to be together.

This description comes from the 2010 Simon Pulse edition.

Synopsis from Open Library.