Grim Tuesday (The Keys to the Kingdom, Book 2) by Garth Nix
Second title in Garth Nix's gripping new fantasy adventure series.
Arthur Penhaligon didn't think he would ever have to return to the strange house that nearly killed him on Monday - the house that contains a fantastical and sinister realm inside.
But the next day brings new challenges - in the form of an enemy named Grim Tuesday, who threatens the safety 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 force him to steal a Sunship, survive a very weird work camp, befriend a bearlike spirit and fight the void Nithlings.
Even after all that, he will still have to venture into the scary Far Reaches for an ultimate showdown.