Leviathan by Scott Westerfeld

Alek is a prince to the Austrian-Hungarian throne and on the run for his life. His parents were murdered by their government, to start a war. Alek flees across Austria with two loyal masters and two commoners devoted to saving his life.

Deryn is a British commoner determined to join the air force and fly. Problem--she's a girl. So she disguises herself as a boy and manages board the airship Leviathan, a hybrid animal controlled by hydrogen and some machines.

Alek and Deryn have common enemies and when forced together, they become allies and friends. But neither knows where that will take them. Or the dangers they'll face.

A steampunk novel with a lot of action and suspense. The first in a series--and the action starts off the story and never quits. Notes at the end explain where Westerfeld got his ideas. I can't wait for the next in the series!