What is the garth nix book lord sunday about?

Lord Sunday is the eldest child of the The Old One and the The Architect, the Mariner and the Piper being the second and third children respectively. He wields the seventh Key, and rules The Incomparable Gardens. He has ultimate authority in the House, but tends to leave the charge of the day-to-day affairs to his deputy, Superior Saturday, who has been corrupt and destructive.

One of the Seven Trustees, Lord Sunday defied the Seventh Part of the Will of his mother and, along with five (Wednesday giving Arthur her key) other Trustees, keeping his demesne and Key for himself rather than yielding it to a human heir.

Though little is known about Sunday, it is known that he opposes the Old One; when the Piper attempted to release the Old One from the binding laid upon him by the Architect Herself, Lord Sunday reportedly cast the Piper into the Void of Nothing, nearly destroying him; however, it is revealed in Superior Saturday that Saturday is the one who threw the Piper into the Void. Lord Sunday is likely to be the most powerful and important being in the House since the Architect's disappearance, given the assumption that his Key is more powerful than the others when outside the House.

Each of the Trustees is afflicted with a deadly sin. Six of the seven have been portrayed, leaving only pride for Lord Sunday.
It is revealed in Superior Saturday that if all the realms of the House were to be destroyed by Nothing (a fictional substance resembling both antimatter and dark matter), the Incomparable Gardens will remain for some time, and will keep the universe around them from collapsing, therefore making the Incomparable Gardens the most important part of the House. The Incomparable Gardens are 'defended' by a variety of bug-like creatures (believed to be Nithlings), mostly giant worms, beetles, and similar. The Gardens' supporting Drasil Trees, which are situated in the four corners of the Upper House, are also defended by beetle-shaped Nithlings. Past attempts to invade have failed due to these creatures, and Superior Saturday's attempts to build a tower tall enough to reach Sunday failed due to the Drasil trees' growth until she was able to slow them down by destroying the two lowest realms of the house with Nothing. Doing so slowed the growth of the Drasil trees that supported the Incomparable Gardens.

Superior Saturday was extremely envious of Lord Sunday and even attempted to take over his reign of the Incomparable Gardens. Aided by a force of her various minions, she was able to enter the Gardens and engage in battle with Sunday's creatures. This is the last part of the House Arthur must claim; given that the aforementioned battle is incomplete until the seventh book, he will probably do so in competition with both Sunday and Saturday, and possibly The Piper.
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