"Sherlock Holmes: The Adventure of the Speckled Band" by Sir Arthur Conan Doyle

Soon to be married, Helen Stoner comes to Sherlock Holmes, terrified that she may meet the same fate as her twin sister, who died mysteriously two years earlier - right after announcing her own engagement. Suspicious of foul play at the hands of her vicious step father, she implores Holmes and Watson to help her. A diabolical plot unfolds.