In Leviathan, we were introduced to a world on the brink of World War One, with an alternate history twist: the allied powers are all "Darwinists," living in societies where Charles Darwin's theories have given birth to a menagerie of engineered lifeforms, from blimp-sized flying war whales to the tiny flechette bats who eat sharp, plane-wrecking steel missiles and crap them on command at enemy airscraft; on the other side are the "Clankers," who use German precision engineering to create a range of terrifying steam-driven mecha and war-machines.
Now, in Behemoth, the two powers fight for primacy in the strategically vital Istanbul, deploying spies, bioweapons, towering iron golems, chemical bombs (parcels of cayenne pepper, flung with deadly accuracy into the mechas' cockpits), and all manner of materiel.
And at the center of the fight remain Alek, the secret heir to the Austro-Hungarian empire, and Deryn, a young girl posing as a boy and working as a midshipman on a great, living British flying ship. Torn apart and scattered to their fortunes in the streets of Istanbul, they ally themselves with anarchist, inter-ethnic revolutionaries who have co-opted the city's ghetto-guarding mecha and plan to depose the sultan, who has become a puppet of the occupying clanker forces. Never sure of whom to trust -- never even really sure of one another -- Alek and Deryn find frustrated romance, heroism, and challenges that tax their ingenuity and resilience to the brink in this page-turning potboiler.
As with other Westerfeld books -- the Uglies series, So Yesterday, Peeps, etc -- Behemoth can be read as a straight-ahead adventure yarn, or it can serve as a jumping-off point for a series of fruitful discussions and investigations into evolutionary theory, history, poleconomy, comparative religion, military tactics, gender, and bioethics. And of course, the fact that it's marketed as "young adult" fiction doesn't make it any less enjoyable for us grown-ups. Highly recommended (natch!).