What Is the Use of a Message Without the Message?

It is obviously an attempt to convey secret information. Why douglas and beerlstone? Clearly because those are words which are 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 enclosing 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