It's for a teenage fiction book called Leviathan by Scott Westerfeld. Some may know Scott Westerfield for his Uglies series but this book and it's sequel Behemoth sit in the "steampunk" genre and are aimed at the teenage market.
For those who don't know what "steampunk" is, here's a definition I found on the wikipedia website:
Steampunk involves an era or world where steam power is still widely used—usually the 19th century and often Victorian era, Britain, that incorporates prominent elements of either science fiction or fantasy. Works of steampunk often feature anachronistic technology or futuristic innovations as Victorians may have envisioned them; in other words, based on a Victorian perspective on fashion, culture, architectural style, art, etc. This technology may include such fictional machines as those found in the works of H. G. Wells and Jules Verne or real technologies like the computer but developed earlier in an alternate history.