Mister Monday is the first novel in the series The Keys to the Kingdom by Garth Nix. It follows Arthur Penhaligon, a twelve-year-old boy who discovers that he is the heir to an otherworldly House and must fulfil a mysterious Will in order to claim it from seven antagonistic Trustees.

The Keys to the Kingdom is a fantasy-adventure book series written by Garth Nix, comprising seven books published between 2003 and 2010. The series chronicles the adventures of a boy named Arthur, who becomes involved with a magical world called the 'House'; he is on a quest to take back the House from seven antagonistic 'Trustees'.