Idea - Social ebook annotation

The idea

While someone is reading an e-book, they should be able to highlight a quote or passage and make a public annotation that other readers can see while they are reading, and likewise they can see annotations that other people have posted. Annotations become comment threads for further discussion. It's a micro-community around this one book, integrated within the content itself. As you read the book, you have an experience that is shared with the other people who have read it or who will read it in the future.

Imagine reading a sentence in a book that is a huge reveal, or poses a philosophical question, and you can immediately see that other people are discussing that exact line, and join the discussion or start your own.

The purest form of this product would be limited to public-domain books, so that anyone could load up the book in the app/browser and read it and contribute to the annotations.

My more general vision would be for the community annotation tool to work for any e-book file that someone would locally load (i.e. their arbitrary file is not being hosted by any cloud server), and anyone else who has the same file locally would also see the public annotations that they've made.

Limitations

I realize that many (most?) people read physical paper books, so the e-reader version of this is not useful for them if they want to keep reading physical books. However, they could still access a social view that lets them see the annotations that other people have posted, and they could also still participate in the discussions and even manually add a new annotation. (This applies to people listening to audiobooks as well.)

I also realize that most people reading e-books are reading them on an established e-reader like Kindle. I don't envision a realistic way of creating something that would apply to existing e-readers. So maybe people would just come to my app to view/add annotations rather than using it as an e-reader if they don't want to.

Spoilers

One tricky part to account for is to make sure that annotations don't contain spoilers for the content ahead. It might not be possible to have a perfect solution here. At minimum, the commenter should be able to mark the annotation as containing spoilers, and other users should be able to flag an annotation as containing spoilers (which would maybe mark it as a spoiler if enough people flag it).