Copyrighted in 1962, The Diamond in the Window is one of several children's books written by Jane Langton.  Set in Concord, Massachusetts, the story is about Eleanor (10) and Eddy (8) Hall, orphaned siblings who live in a grand but shabby Victorian house with their Aunt Lily and her older brother Uncle Freddy.    

The children discover a secret room at the top of the house and...well, you'll just have to listen to find out what happens next!