The Secret Garden by Frances Hodgson Burnett

This timeless classic is a poignant tale of Mary, a lonely orphaned girl sent to a Yorkshire mansion at the edge of a vast lonely moor. At first, she is frightened by this gloomy place until she meets a local boy, Dickon, who's earned the trust of the moor's wild animals, the invalid Colin, an unhappy boy terrified of life, and a mysterious, abandoned garden...