The Keys to the Kingdom: Mister Monday by Garth Nix

The Keys to the Kingdom: Mister Monday is about a kid that is delivered a minute hand from a mysterious man called Mister monday. The kids name is Arthur. The minute hand makes it so the Arthur can heal himself really fast and stop his astma attacks as long as he is touching the key. Mister Monday wants the minute hand back so he sends his minions after the boy. The minions cause a terrible plague on his friends and the city. It is up to Arthur to find out where these mysterious people are coming from and save the city!