Let's cut to the chase: this book is awesome.
Leviathan by Scott Westerfeld (for ages 12 and up) is an alternative history of World War I. It's a steampunk novel. For those of you who don't know, Steampunk is a type of science fiction in which steam power was never replaced with coal. Instead, steampunk fiction takes place in a pre-electical world filled with almost-modern, fantasy technology. And, Leviathan fills that world with powers more fantastical than most: the Darwinists and the Clankers.
The Darwinists are the Allied Powers (the U.K., France, Russia, the U.S., etc). Their power comes from the discovery of DNA way earlier that it's actually discovered. They use this to genetically engineer hybrid animals (that they call beasties) which do everything we're used to machines doing. They have elephant-cars and bat-bullets and whale-airplanes.
The Clankers are the Central Powers (Austria-Hungary, Germany, the Ottoman Empire, Bulgaria). Their technology is machine-based, but it's not like any machines you or I know. Their machines are giant robots, that people ride and power from inside, a little like Transformers or a lot like the mecha from anime.
The story starts right at the precipice of war and alternates back and forth between the point of view of two narrators, Deryn Sharp and Aleksandar (Alek) Ferdinand. Deryn is an English (Darwinist) girl who just wants to fly beasties. But only boys can join the air force. So, there's only one thing to do — pretend to be a boy. Alek is the prince of the Austro-Hungarian Empire (a Clanker power), and he's on the run from the people who assassinated his parents. (This isn't made up; the assassination of Archduke Franz Ferdinand of Austria really caused the start of World War I). And when their paths cross, it could change the course of the war and history forever. I say "could" because the book ends on a cliffhanger with the promise of an action-packed sequel.
If you're worried you don't "get" the technology or that you don't know enough about World War I, don't worry. The book starts slow and explains everything you need to know. (On the other hand, if you're a WWI buff, you might find the beginning a little too slow). The technology is wonderfully fantastical and World War I is the perfect backdrop for Westerfeld's heart-pounding action sequences. So if you’re a fan of history, sci-fi, action or fantasy, you are sure to love Leviathan by Scott Westerfeld.