The Window at the White Cat 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 Window at the White Cat, published in 1910, is a classic mystery novel that features a love triangle as an attorney named Jack Knox falls hopelessly in love with the young Margery Fleming.