From the website:
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. […] 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. It can be applied to a fixed document (paper/essay/book etc.) or to a running blog. Use it in combination with multisite, BuddyPress and BuddyPress Groupblog to create communities around your documents.
All comments require a name and e-mail (the latter is not published publicly) to be submitted and appear in a threaded view. There is also an option for the commenter to submit their website if desired.
Example usages of CommentPress can be found here.
CommentPress Version 3.8.x (also known as CommentPress Core) is available for download at the WordPress Plugin Directory which also contains the tool’s documentation.
CommentPress was originally developed by Eddie Tejeda (Institute for the Future of the Book), and has been updated with the help of Christian Wach, Giant Chair, the Writers Centre Norwich, Trinity College (CT), and the Digital Thoreau project at SUNY Geneseo.