Scott Westerfeld's Leviathan trilogy is an excellent YA series for fans of alternate history or the steampunk genre. Taking place during World War I, the books pit the British Darwinists (who use fabricated animals as their weapons and vehicles) against the German Clankers (with futuristic machines). At the center of the story are two young people: Deryn Sharp, a midshipman in the British service who has been forced to disguise her identity and present herself as a boy, and Prince Aleksandar Ferdinand, an Austrian royal on the run from the very same people that murdered his father, Archduke Franz Ferdinand, at the start of the war. When the Darwinists' new flying whale ship, the Leviathan, crash lands near Aleksandar's hideout, Deryn and Alek meet for the first time and their stories become intertwined.
 
Westerfeld, who also wrote the popular YA series Uglies, has created a unique alternate history and through his well written characters he keeps the story fascinating and engaging. The first book Leviathan was recently followed up by the sequel Behemoth, and according to Westerfeld's website, a third book titled Goliath should be released in October 2011.