Grim Tuesday (Keys to the Kingdom, Book 2) by Garth Nix

Arthur Penhaligon's magical adventures continue as he faces a new grave danger -- in the form of an enemy named Grim Tuesday. More amazing fantasy from best-selling author Garth Nix.

Arthur doesn't think he will ever have to return to the strange house that nearly killed him on Monday -- the house that contains an entire world. But Tuesday brings new challenges -- in the form of an enemy named Grim Tuesday, who threatens the well-being of both Arthur's family and his world. Arthur must retrieve the Second Key from Grim Tuesday in order to save everything -- an adventure that will include stealing a Sunship, surviving a very weird work camp, befriending a bearlike spirit, fighting the voidlike Nithlings, and traveling to the scary Far Reaches for the ultimate showdown.