Grim Tuesday by Garth Nix

The second incredible book in The Keys to the Kingdom, now in hardcover 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.