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.

"The same writing," remarked Holmes, as he opened the envelope,
"and actually signed," he added in an exultant voice as he
unfolded the epistle. "Come, we are getting on, Watson." His
brow clouded, however, as he glanced over the contents.

"Dear me, this is very disappointing! I fear, Watson, that all
our expectations come to nothing. I trust that the man Porlock