YA steampunk. It's almost WWI, but not quite the way you learned about it in history class. The archduke's been assassinated, and his only son, Alek, is on the run in a Clanker, one of the steam-powered machines favored by the Austrians and Germans.
Meanwhile, in England, Deryn Sharp, a girl posing as a boy named Dylan, enters the air service and finds herself on the Leviathan, a monstrous living airship. See, in this England, Darwin has discovered how to manipulate DNA, so they have living machines called Huxleys (or possibly Huxlies; I forget). Leviathan is a whale, and a whole lot more. Before long, Deryn and Alek's adventures merge, and they're sailing off into the sequel.
There's a lot of good stuff here. The Clankers and the Huxlies (or Huxleys) are presented in such detail that you almost believe them, and the human characters are even better, particularly Deryn. And then there's the mysterious Dr. Barlow and her the even more mysterious eggs that she's trying to deliver to Constantinople.
High adventure, air battles, Clanker chases, and I almost forgot to mention the great illustrations by Keith Thompson that are scattered throughout. Fun for all ages, really.