Showing Goliath by Scott Westerfeld

Alek and Deryn are back onboard the Leviathan. The ship is ordered to pick up Tesla, a Russian inventor who has created a machine he claims can destroy half of the world, which he is using as a threat to impose peace. Alek wants to the end the war, so decides to back Tesla politically, as do the Darwinists. Meanwhile Deryn is still pretending to be a boy, though Alek has figured out her true identity, and promises to keep her secret. With stops in New York, California and Mexico, Deryn and Alek encounter adventure and intrigue at every turn, but when a secret German plan to sabotage Tesla's machine leads to a heart-stopping stand-off, as Tesla threatens to fire his weapon, it's up to the two of them to stop him - or face the end of the world for real…