Speckled Band : Easy-to-read (Sherlock Holmes) -- Paperback / softback

Helen Stoner has enlisted the help of Sherlock Holmes to investigate the strange death of her twin sister, Julia. On the night Julia died of "fright", Helen had heard a low whistle and a clanging metal sound. Julia had cried out "Oh, save me, Helen! It was the speckled band" But what was she talking about?