When Fin's guardian, Judge Harlequin Brown is murdered, life as he knows it becomes a whole lot more exciting and dangerous. In the course of one life-changing day, he learns that there is a pathway between his London in 'Somewhere', and an alternate London of a parallel world called 'Nowhere', and that the fates of both worlds are now under threat from a group of rogue Knights, who have the ability to travel between these worlds using their double edged swords.
[[Un-Lun-Dun by China Mieville]] is a similarly dark fantasy novel set in a very different alternate London that I would recommend to those who enjoyed the premise of this story.