Instead of that, he had intended, before his plans were nipped, to send me the clue in this envelope.

He says so in his note.

This would seem to indicate that the book is one which he thought I would have no difficulty in finding for myself.

He had it--and he imagined that I would have it, too.

In short, Watson, it is a very common book." "What you say certainly sounds plausible." "So we have contracted our field of search to a large book, printed in double columns and in common use." "The Bible!" I cried triumphantly.
"Good, Watson, good! But not, if I may say so, quite good enough! Even if I accepted the compliment for myself I could hardly name any volume which would be less likely to lie at the elbow of one of Moriarty's associates.