The Ghost in the Garden by Alisse Lee Goldenberg

Twelve-year-old Sophie Madison has no idea what she's in for when she moves from the big city of Calgary to Stratford, Ontario. And she misses her friends, her apartment, and all her favorite places. Although her parents found a beautiful house with a bedroom she loves, none of them knew it was full of secrets and surprises. When a young girl keeps lurking in the back garden, she begins to suspect her new home is haunted.