Leviathan

Leviathan, by Scott Westerfeld
 
Set in the early part of the 20th century, right before the beginning of World War I, Scott Westerfeld's Leviathan spotlights a world where Europe is divided into two nations. There's the "Darwinist" countries (England, France, Russia) who have broken the DNA code and can now create or clone hybrid creatures as weapons and transport vehicles. Meanwhile, there's also the "Clanker" countries such as the Germany and Austria-Hungary who use steam driven iron machines (like tanks with mechanical stilt "legs" instead of wheels called "Stormwalkers").

When the Archduke and Princess of Hapsburg are murdered one night, their son Alek must flee for his life. At the same time in England, a young girl named Deryn, who poses as a teen-aged boy in order to join the British Air Service, is assigned aboard the Leviathan, a massive whale-like creature used as an aerial warship. After many adventures, and with the onset of Europe on the brink of war, Alek and Deryn find themselves working together.

Westerfeld's story, solidly supported by Keith Thompson's illustrations, is a terrific, fast moving adventure with thrilling situations and believable characterization. But don't take my word for it. Read it yourself! And check out the sequel Behemoth afterwards!
-Ed