Papertrail is a service for taking digital notes for books you read in print in a way that is accessible, sharable, and optionally public-facing. It operates a smidgen like a forum where each book is its own kind of subreddit. It does, though, require having the book in question and actually reading it in order to participate in and contribute to discussion, but that’s not always a bad thing.
https://papertrail.biblish.com