Arthur Conan Doyle 
Valley of Fear 

Sherlock Holmes and Dr. Watson receive a letter from an informant known by the pseudonym Fred Porlock. Porlock is part of Professor Moriarty’s criminal organisation. The letter is written in a numeric code, and Holmes realises that the numbers refer to words in a book, by page and column. They decode the letter which warns that John Douglas of Birlstone House is about to be murdered. After they have deciphered the message, Inspector Mac Donald of Scotland Yard comes to consult Holmes. Mac Donald is astonished when he sees the message, because it has pre-empted his news: a man called John Douglas has indeed been mysteriously killed in Sussex. Mac Donald demands to know the true identity of the informant who predicted the crime, but Holmes does not know it, because he promised not to try and find out who 'Porlock’ really is. Holmes can only tell Mac Donald that the informant works for Professor Moriarty… (Excerpt from Wikipedia)