Make sure "Next" button is always available, so users won't have to look in the sidebar on the left to continue reading at the end of each chapter.
When clicking "Next" button or navigating to the next chapter, the content area scroll should reset, so I don't end peeking the end of the next subchapter after scrolling down the previous one. (Bonus points for actually remembering the scroll position of the last/each content page, so after accidental click on Next I won't reset my progress in the subchapter).