Eons ago, the Architect of the worlds left, willing her power to whosoever was the Rightful Heir. She left behind seven of her most trusted children behind as Trustees of her Will. But the Trustees did not see any need to relinquish their power, and so they hid the Will away and kept the Keys to the Kingdom for themselves. The Trustees, also known as the Morrow Days because each controlled a single day of the week, were slowly twisted by the Keys, and their foibles and weaknesses became magnified. The House that the Architect built was slowly destroyed by their greed, sloth, and single-mindedness.
Eventually, the first part of the Will tore itself free from its binding place and forced itself upon young Arthur Penhaligan, a mortal boy from a slightly futuristic Earth. With its power and guidance, Arthur has been able to gain the keys from four of the seven Trustees. But in using the Keys, Arthur has become closer and closer to becoming a Denizen.
Having just vanquished Sir Thursday, Arthur recieves a letter from Lady Friday: she's abdicating, and her Key goes to whoever can find it first. It's a traaaaaap! Her plan was to have the Piper, Saturday and Arthur fight it out, and then whoever wins will get sucked into an abyss of Nothing. It almost worked, too, except that Arthur figures it out and warns the Piper.
Meanwhile, she retreats to a far off planet to devote herself to her favorite hobby: sucking out the life experiences of mortals. While Leaf tries to stop her, Arthur tries to find Friday's portion of the Will. Friday's realm is a lot of fun: it's where all the paperwork happens in the House, with rivers made of parchment and a printing press so massive that Denizens take their tea time on it.
Arthur is a fantastic main character. He starts out as a very ordinary little boy, constantly asking questions and wishing he could just go home. With every book, he loses more of his naivete and innocence. After years of chronically, nearly fatally, poor health, Arthur has incredible strength of will and determination. And his parents' influence and his own empathy make him polite and fair-minded, always treating his defeated enemies well. His determination to save everyone puts him in a great deal of danger and trouble, but it makes him a joy to read. I really like the morality system underpinning this series.