I recently finished reading Railsea by China Mieville.  I've enjoyed a lot of his other books (Perdido Street Station, The Scar, Kraken, Un Lun Dun, Embassytown) and this is up there as a well written and nicely imagined fantastic setting.  I like the simple monster/adventure plots of this and Perdido Street Station, partially because it feels like a good scaffolding in which to explore the setting without feeling like purely an excuse for setting exposition.

Definitely recommended if fantastically imagined settings and young-adult-style adventure is appealing.