An additional level of privacy between "followers" and "private" would be useful to users who want to invite family or coworkers to the app, but don't want certain books visible to them, while allowing those books to be visible to others.

Inviting family or coworkers to an app tends to come with expectations that if they follow you, you have to follow them back. This can lead to marking books as private that you would otherwise want shared with a subset of like-minded readers, or it can lead to not inviting people to the app as a privacy measure.