Drowned Wednesday

Drowned Wednesday is the third book in the "The Keys to the Kingdom" series by Garth Nix. This book was released in February 2005.
Plot.
Background.
Arthur Penhaligon is a young boy who has gotten involved with the 'House', a magical world. This world comprises seven parts, each containing a 'Key' (powerful magical objects) and a part of the 'Will' (a being that holds the wish of the absent 'Architect'), under control of a villainous 'Trustee'. Arthur is on a quest to defeat the 'Trustees' and fulfill the 'Will'.
In the preceding two books, Arthur has captured two parts of the House.
This book.
Unlike the other Trustees, third Trustee Drowned Wednesday is on Arthur's side and wishes the Will to be fulfilled. The other Trustees have taken away most of her powers for this reason and taken her part of the Will from her, which was hidden somewhere else by a pirate named Feverfew.
At the start of the book, Arthur's friend Leaf is visiting him in the hospital when the room suddenly fills with water and the two are transported to Wednesday's part of the House, the Border Sea. They are separated: Leaf is snatched away by a ship , while Arthur is left behind. He tries to call for help with a magical medallion that his ally the Mariner had given him, but this seems to not work. Eventually he is rescued by a different ship.
After a fight with Feverfew's ship, Arthur reveals his identity to the crew. Among them is the sorcerer Scamandros, who gives him a magical scrying mirror with which he can check up on Leaf. He is then fetched by a servant of Drowned Wednesday, who wants to meet him. Wednesday, who normally looks like a leviathan but temporarily assumes human form, tells him that after part three of the Will is released, she will relinquish her Key to him.
Arthur makes a deal with the 'Raised Rats', a group of anthropomorphic rats, to take him to Feverfew's hideout, which they believe is inside a miniature world in Wednesday's stomach. On the Rats' ship, Arthur uses another one of Scamandros' gifts to communicate with Scamandros and rescue him. The Rats bring them to their submarine, where they find Arthur's friend Suzy. After some navigational difficulties, the submarine is able to enter Wednesday's stomach and the miniature world in it.
Arthur and Suzy, disguised as Rats, find a group of followers of a being called the 'Carp'. The followers bring them to the Carp, which turns out to be the third part of the Will. But then Feverfew appears, who has taken Leaf prisoner. He proposes a fight between him and Arthur where each tries to kill the other in one strike. Arthur goes first but fails, then dodges Feverfew and severs his head. Being a House creature, this does not quite kill him, but Leaf kicks the head into a pool of a substance called 'Nothing', which can kill a House creature. When he dies, the miniworld begins to collapse; Arthur and his friends escape with the part of the Will, but Wednesday dies, poisoned by the miniworld that has entirely turned into 'Nothing'.