The Valley of Fear - Sir Arthur Conan Doyle

The novel starts with Sherlock Holmes receiving a cipher message from Fred Porlock (pseudonym), an agent of Professor Moriarty. After Porlock sends the message, however, he changes his mind for fear of Moriarty's discovering that he is a traitor. He decides not to send the key to the cipher, but he sends Holmes a note telling of this decision. From the cipher message and the second note, Holmes is able to deduce that it is a book-cipher and that the book used for the encryption is a common book, large (with at least 534 pages), printed in two columns per page, and standardized. An almanac fits these conditions exactly. Holmes tries the latest edition of Whitaker's Almanac, which he had only received a few days earlier, and fails; he then tries the previous edition. With this almanac, Holmes is able to successfully decipher the message as a warning that "some devilry is intended against one Douglas," a country gentleman residing at Birlstone House. Some minutes later, Inspector Macdonald arrives at Baker Street with news that a Mr. John Douglas of Birlstone Manor House, Birlstone, Sussex, has been murdered. Holmes tells MacDonald of Porlock's warning, suggesting Moriarty's involvement. However, MacDonald doesn't fully believe that the educated and well respected Moriarty is a criminal. Holmes, Watson, and MacDonald travel to Birlstone House, an ancient moated manor house, to investigate the crime.