I was conflicted when I heard this was "a gripping and brilliantly imagined take on ... [b:Moby Dick|153747|Moby-Dick|Herman Melville|http://d.gr-assets.com/books/1327940656s/153747.jpg|2409320]" (so it says on the cover blurb). On the one hand, Miéville at his best is one of the best writers on the planet. On the other hand, I hated Moby Dick (yeah, I gave it 2 stars - because one would have meant I couldn't finish it).
On the gripping hand, I already panned [b:The Scar|68497|The Scar (New Crobuzon, #2)|China Miéville|http://d.gr-assets.com/books/1320435192s/68497.jpg|731674] because it was too much like [b:Moby Dick|153747|Moby-Dick|Herman Melville|http://d.gr-assets.com/books/1327940656s/153747.jpg|2409320]!
It turns out Miéville is either mocking [b:Moby Dick|153747|Moby-Dick|Herman Melville|http://d.gr-assets.com/books/1327940656s/153747.jpg|2409320], or he's explaining it in terms that make it easy for illiterates like me to understand! In any case, I find the book vastly more interesting than Moby Dick.
Everybody knows that Moby Dick is not actually about hunting whales, it's about the sort of worldview that leads to monomania - about philosophy. In [b:Railsea|12392681|Railsea|China Miéville|http://d.gr-assets.com/books/1321409808s/12392681.jpg|17373771], Miéville turns this on its head: the massive animals hunted by railsea captains, those animals that in prior encounters have removed a limb or two from said captain, are known as Philosophies! Any captain worth his or her salt has a Philosophy (and one or more missing limbs).
Despite being marketed as a Young Adult novel, I don't see it. The protagonist is, presumably, a youngish adult. Not so young that it seems inappropriate that he frequents pubs, though, and he could in any case be fully adult without anything changing.
The world of Railsea is, as always from Miéville, finely rendered and quite unlike anything anybody else has written.