Next/Previous Chapter at bottom of page?

Let me just start by saying I love this plugin.

However, what keeps it from being perfect is when reading a chapter, at the end of the page, I can link to the next post, but not the next chapter of the ‘book’. Only the next chronological chapter that, in a fan-submitted story site, is impossible to land to the next chapter. It may as well be a random post button.

There is a plugin I found that could take posts in a category, and post links to posts in that category, but I can’t put chapters in POST categories.

I’m deathly afraid of altering the PHP on my template for home remedies to this, so if that’s my only option, could someone please explain what to copy-paste, and where?