I picked this book for reading in waiting rooms because (A) it was written for kids in grades 5-9 and (B) I'd very much enjoyed Mieville's first book, an adult novel called King Rat.
Un Lun Dun is set, for the most part, in an alternate London, a place where all the "moil" of the real London goes, the moil being everything that's broken, discarded, or obsolete. Things there are in a turmoil, and the people are looking for "the Shwazzy" to appear and save them. The Shwazzy appears, but things don't go exactly as planned. There's a quest, lots of action, and lots of weirdness before it's all wrapped up.
Mieville's great at coming up with one damn thing after another, and he's also great at coming up with one fantastic image after another. What's he's not so great with is characters. There's not a one in this book that comes alive. I didn't really care if any of them lived or died. Also, there's not a lot of wit and humor. In a book this long, you need that. Or I do.
And speaking of long, the book is nearly 430 pages of small print. I wonder how many kids in grades 5-9 will actually plow through it. I know there were times I was tempted to put it aside, but the situation demanded that I finish it. The ending holds out the promise of many sequels. I found I didn't much care. Probably I'm just old and cynical, but I really wanted to like this book. I didn't, or at least not enough to recommend that you check it out. I liked Mieville's illustrations, though.