Behemoth by Scott Westerfeld

In this alternate WWI steampunk trilogy the Darwinists (Britain, Russia) whose warships are made of fabricated animals, and the Clankers (Germany, Austria-Hungry) who use metal and steam for their machines, are moving closer to all out war.

When last we left our heroes, Deryn, who has disguised herself as a boy in order to join the British Royal Air Force and is going by Dylan, and Alek, the prince of Austria-Hungary whose parent's assassination set in motion the start of World War I, were aboard the air ship Leviathan heading toward Istanbul (not Constantinople).

Now that Austria-Hungary has officially joined Germany and declared war against the Darwinists, Alek and his men aboard the British ship Leviathan, are technically prisoners of war. But they're the only ones who know how to operate the Clanker engines now attached to the Levithan. Alek knows that once the British airmen learn how to operate the engines, they won't have any more use and will be locked up, so they begin to plan their escape.

Deryn continues to be torn between her loyalty toward Alek, and her loyalty to Britain and the Leviathan. She is also constantly in a dangerous position, hiding her true identity. To add even more confusion, Deryn is assisting Dr. Barlow, a women scientist who has created an brand new animal fabrication that is to be presented to the sultan of Istanbul, currently a neutral country, to pull them away from German control. But they may already be too late.

Awesome follow up to the Leviathan. Deryn has several moments when she almost tells Alek the truth, but just can't. She continues to perform above and beyond the boys, and whose quick thinking keeps getting everyone out of trouble. Girl power! Alek is still struggling with the responsibility of being a prince and feels he may have the power to end the war. Both characters continued to develop throughout the book, and I'm looking forward to the final installment!