Goliath (book 3 of Leviathan trilogy), by Scott Westerfeld.
Man, I loved this trilogy. LOVED it.
Remember when I reviewed His Majesty's Dragon and said I disliked books that use real places in a fictional story? Turned out I only disliked it in that book. Among the many joys of Goliath was that it had so many real places and people I knew! The climax took place in the part of New York where I grew up. In the middle of the book they were in California and mentioned buildings I knew and had seen. Real people from that period of history showed up. And man was it one grand adventure.
In the first two books we were introduced to the 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). 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.
Now to poke around Google and see if maybe he's planning to write more in this world. :D
Edit: Three books and a couple months later, I still vividly remember the emotions the beginning of the first book dragged me though. Quite the sign of an outstanding series!
Edit 2: Things learned about Scott Westerfeld while googling: He's not just pro fanfic of his work, he's pro slash! Two thumbs up for him!