The cipher message begins with a large 534, does it not?
We may take it as a working hypothesis that 534 is the particular page to which the cipher refers.
So our book has already become a LARGE book, which is surely something gained.

What other indications have we as to the nature of this large book?
The next sign is C2.

What do you make of that, Watson ?" "Chapter the second, no doubt." "Hardly that, Watson.

You will, I am sure, agree with me that if the page be given, the number of the chapter is immaterial.

Also that if page 534 finds us only in the second chapter, the length of the first one must have been really intolerable." "Column!" I cried.
"Brilliant, Watson.