If you're looking for some good young adult reading material, this is a fine series. Westerfield's setting is a re-imagining of the world at the time of World War I, but with the two sides armed with fantastical steampunk technology. The British and their Darwinist allies use fabricated (genetically engineered) creatures, such as living airships, while the Clanker Germans and Austrians rely on sophisticated machinery.
Behemoth, the second book in the series, continues at the same rolicking pace as the first, this time in the Ottoman capital of Istanbul. The story's two protagonists, a young British airman (who is actually a girl in disguise) and a deposed teenage heir to Austria's throne, are both appealing characters, and play off one another in a fun way. Westerfield does a decent job of blending the real-life history of World War I with his own world creation, using the ideological differences between the Darwinist and the Clanker powers as part of the story's backdrop (one wonders if his version of America, which apparently follows both paths, will feature in the next book). However, compared to its predecessor, this is a more straight-up adventure story, and I thought that a bit of the tragic historical resonance that I liked about Leviathan got lost among the gadgets and creatures.
Still, the young target audience is unlikely to mind. If you happen to be a parent looking for reading material for your children, there's no profanity or sex, and I can't say that the violence would get more than a PG rating. Also, the audiobook reader does a pretty good job with voices. His portrayal of Deryn's Scottish accent and Alek's Austrian one are now part of the charm for me.