Superior Saturday by Garth Nix

(Keys to the Kingdom series) Allen and Unwin, 2008.
(Ages : 10+) Saturday's adventure in the Keys to the Kingdom series has been much anticipated, and does not disappoint. Arthur is in receipt of the fifth key, but has realised that the Nothing is growing ever more rapidly. He tries to find help, firstly from Dr Scamandros, who makes Arthur realize that he is growing bigger and stronger, and then from the Old One, but none can answer his questions. Using the fifth key to go through the mirror to the citadel, Lord Arthur is held back a little by Scamandros holding onto his shirttails. But once there, they are galavanised into action. Suzy has taken on the mantle of general, wearing the clothing which denotes her status, and she is amazed at the powerful, strong man before her. Lord Arthur is expected to save the House.
Nix always creates a most magical place where ideas, words and descriptions meld into one, making a story line at once easy to follow, but demanding that the readers always use their imaginations, taking hold of the ideas presented, using their intelligence to make sense of the world created before them. Arthur and his allies must hold out against the increasing power of Superior Saturday and find the sixth key to stop the destruction happening all around them.
The many followers of Arthur's quest will be eager to read this and then the seventh and last in this excellent series. For those impatient to find out more, then the website,  can be trawled.
Fran Knight