This image is the cover for the book The Valley of Fear, Classics To Go

The Valley of Fear, Classics To Go

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 MacDonald of Scotland Yard comes to consult Holmes. MacDonald 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. MacDonald 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 MacDonald that the informant works for Professor Moriarty… (Excerpt from Wikipedia)

Arthur Conan Doyle

Sir Arthur Ignatius Conan Doyle KStJ DL (22 May 1859 – 7 July 1930) was a British writer best known for his detective fiction featuring the character Sherlock Holmes. Originally a physician, in 1887 he published A Study in Scarlet, the first of four novels and more than fifty short stories about Holmes and Dr. Watson. The Sherlock Holmes stories are generally considered milestones in the field of crime fiction. Doyle was a prolific writer; his non-Sherlockian works include fantasy and science fiction stories about Professor Challenger and humorous stories about the Napoleonic soldier Brigadier Gerard, as well as plays, romances, poetry, non-fiction and historical novels. One of Doyle's early short stories, "J. Habakuk Jephson's Statement", helped to popularise the mystery of the Mary Celeste. (Wikipedia)