Tom's Midnight Garden By Philippa Pearce

When his brother catches measles, Tom is sent away for the summer to stay with his uncle and aunt and is thoroughly fed up about it. What a boring summer it's going to be. But then, lying in bed one night, he hears the old grandfather clock in the hall strike the very strange hour of 13 o'clock. What can it mean? As Tom creeps downstairs and opens the door, he finds out...a magical garden, a new playmate, and the adventure of a lifetime.