Action! Adventure! War! A young prince, hidden by his faithful servants! A girl disguised as a boy in the Royal Navy! Steampunk! Flying whale airships!
An alternate history of the outbreak of World War I -- noticeably altered by the Clanker (steampunk) and Darwinist (err -- steampunkish sort of genetic engineering?) war machines. The postscript about how it diverges from reality is rather more interesting here, where it's "this is where I stole stuff," than in historical books where it's "this is where reality had to give way."
Alek is the only son of Franz Ferdinard and his morgantic wife. Loyal servants smuggle him off in the night, and only when abroad do they reveal that his parents were murdered --dying of poison in the night. Now they need to get Alek away. Considerable amounts of sneaking about Europe trying to get away from people who want to murder Alek as well.
Deryn, disguising herself as a boy, goes for the tests as a midshipman. One test involves flying on a Medusa, the simplest form of "hydrogen breather" -- a living hydrogen balloon, in effect. Except that when a storm blows up, she has to cut the tether to save herself, but that leaves her flying over England. The airship Leviathan picks her up, but can't send her back; they are heading to Europe. They end up testing her onboard and making her a midshipman on the spot; the tests are supposed to be administered on the ground but they can put another one to use. After they pick up a boffin for a secret mission, they are off flying over Europe -- on a collision course with Alek's story.
Lots of fun adventure. I felt that Deryn was a girl just to have an excuse to have a girl in the story, but perhaps something will come up with in the next -- because this book, while resolving its own storyline, has much that will go on to Behemoth, and its ending -- well, it's not quite Two Towers, but it's as much as lead-in to the next book as The Fellowship of the Ring.