CommentPress is an open source theme for the WordPress blogging engine that allows readers to comment paragraph by paragraph in the margins of a text. It was developed by the Institute for the Future of the Book. Their words:
Annotate, gloss, workshop, debate: with CommentPress you can do all of these things on a finer-grained level, turning a document into a conversation. It can be applied to a fixed document (paper/essay/book etc.) or to a running blog… Over time, we (and hopefully the community) will make improvements and add new features and extensions, all of which will be documented here on this site.
The CommentPress site is presented in “document” mode. If you click through to any of the pages listed in the Table of Contents you’ll see the commenting in action. If you go to the Examples page you can link to some examples of CommentPress in action.
This is something worth watching and trialling.