Leviathan by Scott Westerfeld

I really liked this book.  It's cool steampunk, sci-fi, (alternate) historical, adventuresome fun.  Alek is the son of Franz Ferdinand, the murdered Archduke whose death led to the start of World War I.  Deryn is the rough and tumble daughter of an airman, who has disguised herself as a boy and lied about her age to pursue a life in the British air force. 

It's 1914, but not the 1914 we know.  In this alternate history, the British have taken Darwin's science and developed genetically modified creatures to power their lives and war machines.  The Leviathan of the title is actually a genetically modified whale, a massive, living airship, with a menagerie of other modified creatures that provide power, defense and maintenance.  The Germans & Austrians (Clankers) have developed machines the likes of which we've never seen.  Huge walkers with 2 to 8 legs, iron precursors to the Empire's attack vehicles in the Empire Strikes Back.

The two main characters have amazing adventures on their own, but when their paths cross, things start to get really interesting.  War is brewing, and they are caught up in the intrigue, socio-political upheaval and battles of a pivotal time in history.  This is just the first chapter in an epic tale (more volumes to come), with adventure, history, science and politics that will keep you turning pages and anxious for the next book in the series.  Highly recommended.