I think it would be interesting to include books in Medium's Partner Program. Maybe Medium could create one tier for articles, another different priced tier specifically for books, and another for books and articles?
You could also allow customers to purchase said books, though the subscription model is working quite well in other places (Netflix, Disney+), so for a user to buy their books doesn't seem very necessary. Or, both options could be offered, kind of like Audible.
On that note, any thoughts on whether Medium might offer content creators a place to create Podcasts?