Since you already sell EPUBs, I'd like to propose a paid feature.

Background:
Because this book is so good, I really want to read it from start to finish, and I believe many people feel the same. However in reality, that never happens, because I can never find such a big chunk of time. What typically happens is when I need to learn something, I come to read on that specific thing.

Proposal:
As a signed-in user, I wish to be able to mark a lesson as "studied" or "have read", which serves two purposes: 1. it's a motivation for me to read all the lessons and "check them all off" 2. help me organize which lessons have been read, especially when I jump to different topics without following any orders.

It could be implemented in multiple ways, for example:

• add a button for user to "mark this lesson as read"
• mark a lesson as read when user clicks "next lesson"
• mark a lesson as read when user's browser tab has focused on this tab for more than x minutes

These behaviors could also be configurable as user preference.

Depending on the implementation, a lesson could even be read multiple times, for example, a ✔ is shown on the page to indicate it's been read, and ✔✔✔ shows if it's been read for 3 times. Clicking or hovering the symbol could show the date it happened, and the duration of the session, etc.