I'm honestly a complete noob to tabletop RPGs, having only read the manuals for DnD 4th edition and never having played, but I'd like to know if an RPG or setting for one exists for China Miéville's book Un Lun Dun. It's a personal favorite, and I think it would make for some great gameplay. If not, how would one go about creating a setting and in which RPG, and how difficult would this be?
I would also recommend reading Neverwhere which I consider "the original". If you want something more generic GURPS is worth a look. Since Un Lun Dun can contain close to anything as long it is absurd enough you may use several of the GURPS extension books from Blood Types to Black Ops.
Also the Kult RPG might be worth a look. There are many parallels to Un Lun Dun. Kult is not suitable for minors, though.
There is not anything official/published RPG-wise for Un Lun Dun. There is some adaptation of content for Mieville's world of Bas Lag for D&D 3.5e in Dragon Magazine # 352, one of the last "real" Dragon magazines. China Mieville talks a lot about his playing Dungeons & Dragons and how it informed his novel writing.
Un Lun Dun, however, is more childen-urban-fantasy driven and probably not a good fit for the crunch of D&D. As there's no published game for it specifically you'd have to adapt it yourself, but Little Fears, the role-playing game of childish terror, would be the best direct fit. In a pinch you could probably use Underworld, Monsters and Other Childish Things, or even Dresden Files if forced to. If you want a free one, Neverwhere d6 is based on the Neil Gaiman book of the same name, and Un Lun Dun is nothing if not Gaimanesque.
Un Lun Dun is very fanciful - there's conflict/combat but it's pretty abstracted, so if you're rolling the content yourself really any rules-light game would serve.