An ebook written by "Sir Arthur Conan Doyle". Here are some sample content from the eBook,
"Why do you say 'in this instance'?"
"Because there are many ciphers which I would read as easily as I do the apocrypha of the agony column: such crude devices amuse the intelligence without fatiguing it. But this is different. It is clearly a reference to the words in a page of some book. Until I am told which page and which book I am powerless."
"But why 'Douglas' and 'Birlstone'?"
"Clearly because those are words which were not contained in the page in question."
"Then why has he not indicated the book?"
"Your native shrewdness, my dear Watson, that innate cunning which is the delight of your friends, would surely prevent you from inclosing cipher and message in the same envelope. Should it miscarry, you are undone. As it is, both have to go wrong before any harm comes from it. Our second post is now overdue, and I shall be surprised if it does not bring us either a further letter of explanation, or, as is more probable, the very volume to which these figures refer."
Holmes's calculation was fulfilled within a very few minutes by the appearance of Billy, the page, with the very letter which we were expecting.