There is no story to find despite what he says - that's the end of the quest. Perhaps it's supposed to be a reference that I didn't catch but either way the ending is up to your own imagination. Given Arche and Sana's personalities and the availability of the book, they'd have probably returned and read it to him afterwards so he passes in peace. You can only access that bonus quest from a prologue clear save (correct me if I'm wrong), so it's greyed out for the most part.
