Goliath (book 3 of Leviathan trilogy), by Scott Westerfeld.
Man, I loved this trilogy. LOVED it.
In the first two books we were introduced to this steampunk world, where the "Clankers" (those who believe in engineering/machines) were at war with the "Darwinists" (those who used fabricated animals for everything -- airships made from the genes of whales crossed with jellyfish and other things, a whole ecosystem living on and in them, etc). World War 1 in our time. The two main characters were a girl who was pretending to be a boy so she could join the Brittan's "air force" and a boy prince who had to flee from his home country.
Through the first two books the two characters had many adventures in the prince's quest to end WW1, and the girl fell in love with the boy. So I was a tad worried about the third book, I thought it would be all ~True Love~ and romance and all that. I don't want to give anything away, but I shouldn't have been worried at all. It was great!
I'm not one who usually seeks out action-adventure books, but this one was nonstop action and I loved it. Sometimes I even got tired at all the running around and nonstop action the characters were going through! Silly, but I felt drained, like I had done all those things as well.
I loved the characters, from the main ones to tiny minor ones. I LOVED the world/setting so very much. The writing was great. I don't think there was one thing I disliked about the trilogy as a whole... except that it ended. I actually said "Noooo!" out loud as I finished the last page. Highly recommended!