The Valley of Fear

The Valley of Fear is a mystery novel by Sir Arthur Conan Doyle and was the last of the four Sherlock Holmes novels written by the author.
At the beginning of this novel Sherlock Holmes receives a message from Fred Porlock, an agent to Professor Moriarty. Porlock occasionally sends Holmes insider information.
Moriarty is blameless in the eyes of the law but Holmes knows him to be "the controlling brain of the underworld." Together Holmes and Watson decipher Porlock's message as indicating that a man named John Douglas residing at Birlstone is in danger.
Inspector MacDonald of Scotland Yard calls upon Holmes to ask for his help and informs him that Mr Douglas of Birlstone Manor House has been murdered that morning. Sherlock Holmes tells MacDonald that since he received an alert from Porlock it is probable that Professor Moriarty's influence exists in the matter.