If you're familiar with China Mieville's books, then Un Lun Dun is very much like Perdido Street Station, but with less challenging language, and a far more likable hero. The world of Un Lun Dun (unLondon) and its inhabitants are just as creative as those in Mieville's other books, albeit somewhat less horrific. (Interestingly, I noticed no obvious pro-communism message in this novel.)
If you've never read a book by Mieville, then Un Lu Dun is like Neil Gaiman's Neverwhere, as written with the type of wordcraft on display in Jasper Fforde's Thursday Next series.
If you've haven't read those books either, well, if you liked Harry Potter or The Golden Compass, you'll like Un Lun Dun.
An all around great book. I don't want to say more because I don't want to spoil a moment of discovery. My only criticisms are that the book too often read as little more than a series of barely connected (but still fascinating) vignettes, and that I wouldn't mind reading a novel where the government is not populated by evil schemers and morons.
Buy Un Lu Dun at Amazon.