Behemoth by Scott Westerfeld

This is the second book in the Leviathan trilogy.  Deryn and Alek are still trying to keep the peace amidst the impending war.  The tension between the two is growing, too, as Deryn gets close to revealing her identity (there are some creatures who know she's a she).  There are more machines, Clankers, as it were, too.  It's a good read, but is a bit anti-climactic.  I'm hoping the third book in the series, Goliath, is better. 

Westerfeld, Scott. Behemoth. New York: Simon Pulse, 2010.