The Valley of Fear by Arthur Conan Doyle

It is one of many stories about Doyle's famous detective, Sherlock Holmes. At the beginning of this novel, Holmes gets a message from one of Professor Moriarty's agents—Fred Porlock. From time to time, Porlock sends Sherlock inside information about Moriarty's endeavors.

Despite the fact that the law sees Moriarty as innocent, Sherlock firmly believes him to be the puppet master of a criminal underworld. Sherlock enlists the help of his friend and sidekick, Dr. Watson, to decode Porlock's message; from it, they learn that the life of John Douglas, who resides at Birlstone, is in danger.