The Secret Garden by Frances Hodgson Burnett

THE SECRET GARDEN tells the story of Mary Lennox, a spoiled, contrary orphan, who is sent to live in her uncle's manor in Yorkshire. She is left to herself by her uncle Mr. Craven, who travels most of the time, trying to escape from memories of his wife. The only person who has any time for Mary is the chambermaid Martha. It is Martha who tells Mary about the walled garden, Mrs. Craven's favorite garden, which nobody has seen the inside of since she her death; Mr. Craven locked it and buried the key.