The Secret Garden by Frances Hodgson Burnett.
When Cholera takes the parents of Mary Lennox, she is shipped from India to England to live with her Uncle Craven. Archibald Craven's house is dark, drafty, with over 100 rooms built on the edge of the moors. Mary finds that her Uncle does not wish to see her, which is fine with Mary as she herself is rude and spoiled. While walking the gardens the next day, Mary notices that there is a area in the garden surrounded with a high stone wall and no doorway. Dickon, brother of a house maid, tells her of the garden behind the wall. By the path, the raven unearths the hidden key so that Mary and Dickon are able to enter the walled garden to find it overgrown and neglected. Inside the house, she finds that Archibald has a son named Colin, who is crippled and as spoiled as she. Together these three work to make the secret garden their own world.