It would be nice to have hyperlinks to discussion, explanations and clarifications about every sentence.

Note that this is very different from a hyperlink to some "related" content that leads you on a tangent.

Imagine reading a book (about something complicated, perhaps math) and immediately when you don't understand something you can tap into discussion/QA about the exact sentence you just read.

Of course it would only work for books that are read frequently enough to build that bank of context, but I imagine for many common topics it would work.