Mister Monday, by Garth Nix, 2003. The Keys to the Kingdom, Book One

Arthur Penhaligon ought to have died from his gym class asthma attack. Instead, he has a hallucination of a bedraggled butler named Sneezer and an idle aristocrat named Monday giving him a metal key which restores his breathing. When foul-smelling, dog-faced Fetchers invade his school, and people start dropping ill with Sleepy Plague, Arthur begins to realize what's been thrust upon him. This extraordinarily inventive middle-grade fantasy suggests shades of Oz and Wonderland in its elaborate depictions of the House governed by Monday and the Morrow Days. But its premise is much more ordered and deliberate, leaving a hearty appetite for sequels. Strongly recommended.