Leviathan trilogy

In an alternate 1914 Europe, fifteen-year-old Austrian Prince Alek, on the run from the Clanker Powers who are attempting to take over the globe using mechanical machinery, forms an uneasy alliance with Deryn who, disguised as a boy to join the British Air Service, is learning to fly genetically-engineered beasts.

Alek and Deryn encounter obstacles on the last leg of their round-the-world quest to end World War I, reclaim Alek's throne as prince of Austria, and finally fall in love.