Kell is a interdimensional traveler from a world that uses more magic than ours does--but not so much it destroyed itself, like another one did. Lila is a pickpocket who wants more than anything to be a pirate--but she's a girl. When Kell finds himself with an artifact from the magic-destroyed world, then moments later loses it to Lila's sticky fingers, they embark on a journey to save all the other worlds (and, not incidentally, free Lila from hers). 
 This is a deeply imaginative work that reminded me strongly of Neil Gaiman's Neverwhere, except without the quirkiness or humor. I loved the idea of parallel Londons that have nothing else in common--not even the geography of the rest of the planet. I also loved Kell and Lila's relationship, which might have been fraught, but wasn't in any way sexual or romantic. 
 The worldbuilding was lovely, and I'd love to see more of these worlds--especially the one we never get to actually see.