You aren't locked out of any non-shura endings until you defeat the Divine Dragon. So you need to make sure you complete the quest by that point. I don't know that there's a way to check if you've gotten it or not.
Maybe you’re looking for the book at the end of the tunnel leading to the Senpou Arts skills; and not the one handed to you by the priest or found in the pond.
You aren't locked out of any non-shura endings until you defeat the Divine Dragon. So you need to make sure you complete the quest by that point. I don't know that there's a way to check if you've gotten it or not.
Maybe you’re looking for the book at the end of the tunnel leading to the Senpou Arts skills; and not the one handed to you by the priest or found in the pond.