On the eve of the first World War, Deryn disguises herself as a boy to join the crew of a Darwinist airship, while Alek (fictional son of the Archduke Franz Ferdinand) is forced into hiding by his parents' death. 
 This is the story of World War I, of huge forces smashing against each other as millions upon millions of lives are lost. But this time, the conflict is re-imagined as being between the Darwinist forces - who manipulate the "life threads" of animals to create fantastical war machines - and the Clankers - who build beast-like machines on legs. 
 The book gets classified as Steampunk, and it certainly is, but adding the organic construction aspects to it made it into something new. Is Fleshpunk a thing? Evopunk? 
 The plot is a fairly straightforward adventure story, though it does hint to the deeper politics in the background. The balance is pretty perfect to give the impression of the setting, but without bogging down the story. 
 The book is also classified as Young Adult, which I don't think is quite accurate. It's not a book for little kids, certainly, but I think it would be perfect for someone around 11-14. The characters, who are meant to be around 15 years old, act and think more like 11-12 year olds. 
 That said, the world building and the plot had more than enough to keep me entertained. And I really appreciated that Westerfeld doesn't hold back on terminology - military equipment and ranks are properly named, for example. I could see 11-12 year old me absolutely devouring this book.