Lady Friday by Garth Nix: The Keys to the Kingdom Series

I think what I appreciate most about this dark series is that it takes you to this crazy world and you can't predict how it will spin out.  Garth Nix is quite the story teller, he takes us into this dark world with these incredible challenges to capture the keys of the kingdom.  This time Arthur's adversary is Doctor Lady Friday who kidnaps elderly from Earth to steal their memories and put them in an eternal coma.  Arthur battles to save the day.