Sunscribe to Someone’s Marginalia Feed
Reading a book and seeing someone’s marginalia gives a whole new experience to reading the book.
Have a service that extracts your highlights and notes you make while reading ebooks. You can also input marginalia of real books you read. You then publish the feed to your marginalia and others can subscribe to it and see it on their ebook reader as they read the book. They can subscribe to marginalia from multiple people and see all of them while reading the book.