Leviathan by Scott Westerfeld
I haven't read Westerfeld's the Uglies trilogy, so I can't compare Leviathan to his previous works. But I did thoroughly enjoy the book, despite the fact that it's the first of a series that has just come out and now I have to wait forever for the next volumes.
As a young adult, I don't know if I would have enjoyed the steampunk atmosphere, but I do now. As a kid, I tended to want my books to be pure fantasy or pure sci/fi, but Leviathan has an interesting blend of machinery and nature.
The alternate history is also interesting, especially since I know at least a little bit about WWI. Westerfeld notes that there wasn't really a son involved in the murder of Archduke Ferdinand and Princess Sophie, but Westerfeld nicely sticks Aleksander into the story
I did have some difficulty conceptualizing the Leviathan itself, though. It's hard for me to think of a living ship, a whale-creation that is both alive and mechanized, one that contains an apiary within its belly. I'm also not sure I liked the Darwinists use of animals...I think I preferred the Clankers with their non-sentient machines.
So that I remember when book 2 comes out:
Deryn Sharp, 15, posing as Dylan Sharp, new mid-shipman on the Leviathan; on her first day, even before her middy exams, she was blown away while flying a Huxley, a jelly-fish-like single flier
Jaspert Sharp, Deryn's older brother, and an airman in the corps, helps Deryn disguise herself and study for the exams
Aleksander Ferdinand, 14 to 16 or so, son of murdered Archduke Ferdinand and Princess Sophie; Sophie was not deemed royal enough blood, so the emperor disinherited Alek upon his birth--the archduke achieved a recognition from the pope, declaring Alek heir to the emperor's throne
Count Volger and Otto Klopp--Alek's retainers (fencing master and chief mechanic, respectively), who rescue and support him as he escapes from the Serbs and other enemies out to kill him
The Leviathan docks in Hyde Park, London, to pick up Dr. ? Darwin Barlow, ancestress of Charles Darwin, and one of the original designers of the Leviathan and many other fabricated beasties. Dr. Barlow is on a mission to deliver fabrication eggs to the Ottoman Empire. Even though England is not officially at war during the course of the novel, the Leviathan is shot down over Switzerland, and lands on a glacier which is devoid of any life. The Leviathan can repair herself, but she needs enough food sources to rebuild her energy stores.
At the same time, Alek has been wakened by Volger and Klopp and hustled into a mechanical walker, supposedly as part of his military training. Only later does he learn that his parents have been killed, and Volger and Klopp are following the archduke's final orders to protect Alek. The only way to get him to safety is to flee across Austria to neutral Switzerland, to a glacial castle the archduke and Volger have prepared for just such a contingency. The Austrians see the Leviathan crash, and Alek ventures out to help.
The book ends with the crew of the Leviathan cobbling parts of the Austrian walker to the Darwinist ship, and defeating German forces which have discovered them. They have struck a bargain with the Austrians, not yet their enemies at war, to take them to the Ottoman empire, where Alek and his retainers will disappear.