The Moonstone

According to Wikipedia: "The Moonstone (1868) by Wilkie Collins is a 19th-century British epistolary novel, generally considered the first detective novel in the English language. The story was originally serialized in Charles Dickens' magazine All the Year Round. The Moonstone and The Woman in White are considered Wilkie Collins' best novels. Besides creating many of the ground rules of the detective novel, The Moonstone also reflected Collins' enlightened social attitudes in his treatment of the servants in the novel."