I just finished Un Lun Dun a couple hours ago. I found it engaging from the beginning. I think Mieville has done some really interesting things in his book. It reminded me of a lot of other other books, but in a good way.
A laundry list:The first and most obvious is Neverwhere by Neil Gaiman the whole UnLondon of Mieville is very like Gaiman's London Below. Richard Mayhew becoming invisible to people in London Above because of his association with the people from below is like Deeb being nearly forgotten by her family and friends while she's away because of phlegm.
I did think it was too bad that Zwazzy was so quickly essentially written out of the novel (that's an admittedly a bad writing choice--similar to what Orson Scott Card does with the real estate agent in Homebody.) Nevertheless, Deeb is like Sophie in Howl's Moving Castle,--she's not the one who's supposed to be the hero/savior, but she's the one whose intelligence and bravery solves things anyway.I thought the Wraithtown was a lot like the City of the Dead from Brockmeier's A Brief History of the Dead. It was especially similar given that people either go to Wraithtown or sometimes go straight to the other place. UnLondon is also like Narnia in that one route to get someone there may not work to get back there the next time.
Finally, in a really far reach, Un Lun Dun is like Watership Down in that each of the characters on the good team contributed their part. I don't know that I'll read ULD every other year and get teary-eyed like I do with Watership Down, but Deeb will definitely be in my list of strong, clever, admirable female characters along with Sophie from Howl's, Coraline from Coraline, Lyra from the Dark Materials Trilogy, Thursday Next, Ruth Thomas from Stern Men and others.