Leviathan, by Scott Westerfeld. An alternate timeline in history, in which World War I is fought between Clankers and Darwinists, between giant mecha suits and genetically engineered beasts.
With his parents assassinated, Prince Aleksandar of the House of Hapsburg is forced to flee from the combined armies of Germany and the Austro-Hungarian Empire, piloting a two-legged Iron Walker with the aid of his household servants.
Deryn Sharp has a dream: to enter the Air Service and work onboard one of the British Royal Navy's airbeasts---a flying ecosystem fabricated from the "life threads" of a whale, among many other strange creatures. To do this, she'll have to hide her gender and pretend to be a boy---but getting caught would mean her execution.
When these two forces find themselves trapped in the Swiss Alps, they may have no option other than forming an alliance to survive.