Leviathan by Scott Westerfield

Welcome to a world on the brink of war. Meet a Stormwalker - think Tank with a capital T - and a Leviathan - think Whale turned into a machine with other animals keeping it alive. Meet a girl disguised as a boy so she can be in the Air Force, and a prince, unable to claim his throne, hiding from his enemies on an ice-covered mountain. Scott Westerfeld bring all of these elements together to create an adventure story which has a base in history [there really was a war across Europe which began in 1914] and many elements of fantasy [whales really don't fly!] It's a world in which the characters must decide whether to be friends or enemies, and aren't sure who can be trusted. If you like this book as much as I did, you'll look forward to reading the next books in the series, Behemoth, and Goliath.