I love alternate London stories. Neil Gaiman's Neverwhere is a favorite as well as Simon R. Green's "Nightside" PI stories. None of these are quite suitable for young adults, however.
Mieville came in to fill the gap with Un Lun Dun, a story about a
contemporary girl, Deeba, who discovers an alternate 'steam punk' era
London which is threatened by Smog. If Un Lun Dun can't solve their
problems, current day London may also be affected.
Deeba comes to Un Lun Dun with her friend Zanna, who was supposed to be
the hero of the tale. But, Zanna is injured and they must return to
Realizing the peril they are in, Deeba (the
funny sidekick) returns to Un Lun Dun to try and help. Our heroine
shows resourcefulness, compassion, and a lot of guts.
really intended for grades 5-8, but it's also excellent for older
readers as well. While it's definitely derivative, it's also good fun.
Rebecca McFarland Kyle, June 2012