What can't we see, even when it's right in front of us?
We must "compare notes," but what kind of notes? What if it's musical notes?
I think sound has something to do with this. The sounds from the chapters pages, the odd names of the songs. I think it deserve some looking into.
...also I think we should look at ryukaki's messages a little more closely, because he phrases things in really awkward ways and it could contain a clue.