Ever have one of those books that you can't wait to read, but you don't want the story to end? 
 This is one of those books. 
 Actually it's the entire series. It makes me sad to think I will no longer be on a journey with Alek and Deryn. But the way everything concluded? So. Brilliant. 
 The airship Leviathan has picked up a new passenger, one Nikola Tesla, an intelligent, deranged scientist. The War is escalating and Alek feels he's the only one who can stop it (with Tesla's help), but their ship is moving further and further away from Europe, en route to New York (because everything cool is in New York). 
 There's a bit of controversy on board, as always. Secrets are revealed and random characters from the past show up, and I won't discuss any of it because the entire book is one big spoiler. 
 And of course, we can't go without mentioning the alternate history thing. In the afterward, Westerfeld goes into how much of it is actual fact (namely, most of the characters) and how much he made up (ie, a lot). And just the way these things are interwoven is brilliant, and how many times can I use the word "brilliant" in one review before you get tired of it? 
 Seriously, read this series.