The Secret Garden by Frances Hodgson Burnett was published in 1911. The room behind the closed door, which Matilda refers to, belongs to Colin. Colin is the son of Mr Craven, who has taken on the guardianship of Mary. Mary and Colin are kept a secret from one another until Mary's curiosity encourages her to investigate the room.
The mysterious door leads to the secret garden where Colin's deceased mother would spend much of her time. Although much of the garden seems to be dead, Mary discovers that some of the plants have survived and she decides to tend them and bring them back to life.