After my post reached the first page of Hacker News, I realized I had no way of knowing whether people was actually reading the article or just bouncing.
After tinkering the entire weekend, I came up with a solution to track the time spent on each page, without cookies and without risking any privacy violations. It also takes into account open but not on focus pages.
Now I can tell for sure that people spent on average 20 minutes reading my guide on how to write a programming book.