@Melooon Will users be able to follow books to get e-mail notifications when new entries are posted? (One tangential idea is integration with letters.melonland.net. For example, you can attach a book to a newsletter or create one on the fly, and then when you post, there's a checkbox default checked to post to newsletter too).
Do you have mockups or wireframes or sketches? To the extent that pre-feedback would be helpful, I'd love to see how this might manifest. I ask chiefly because I'd love to help ideate how threaded comments on books, sub-books, posts could work in a way that integrates with the forum re: your communal / "overhear" point.