Mister Monday, by Garth Nix

Garth Nix is one of my all-time favourite authors - I have read all of his stuff! Mister Monday is about a 12-year-old boy called Arthur who, after having an asthma attack that nearly killed him, is magically dragged to the centre of the universe and informed he is the Rightful Heir of the House. All of a sudden, his asthma has disappeared for good and he has become ruler of a whole other world. It sounds great, except for a few things...

The Trustees of the house (the Morrows Days) won't give it up easily, and in each dimension of the House, something criminal or sinister lurks there. Oh, and they want to destroy him. Plus, there's also the part about MAGIC BEING REAL. Who knew? Can Arthur defeat the Morrows Days and bring peace and order to the House and Earth? Or will something far more powerful and evil get to him first?

This is the first in a seven-book series called Keys to the Kingdom. The series takes place over the course of three weeks, and each adventure starts on a new day of the week.