The Bat by Mary Roberts Rinehart

Mary Roberts Rinehart was a prolific writer that is often referred to as the American Agatha Christie.  Rinehart's mystery novels are still treasured by millions of readers today and she is the source of the famous phrase "The butler did it."  Rinehart's most famous books include The Circular Staircase, The Bat, The Case of Jennie Brice, and The Door.

The Bat, published in 1920, is a novel that tells the story of a wealthy spinster named Cornelia Van Gorder and her guests as they investigate a mystery at their summer home.  This was later turned into a classic play that became a Broadway hit and was based off The Circular Staircase.