The Valley of Fear

Sherlock Holmes receives a cipher message from Fred Porlock, a pseudonymous 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 standardised. An almanac fits these conditions exactly.