The Keys to the Kingdom Series Novels

Lord Sunday (The Keys to the Kingdom 6) by Garth Nix
Lady Friday (The Keys to the Kingdom 5) by Garth Nix
Drowned Wednesday (The Keys to the Kingdom 3) by Garth Nix
Grim Tuesday (The Keys to the Kingdom 2) by Garth Nix
Mister Monday (The Keys to the Kingdom 1) by Garth Nix