O'Reilly has launched the Open Feedback Publishing System, which allows readers to comment on in-progress works.
Here's an excerpt from the announcement:
Over the last few years, traditional publishing has been moving closer to the web and learning a lot of lessons from blogs and wikis, in particular. Today we're happy to announce another small step in that direction: our first manuscript (Programming Scala) is now available for public reading and feedback as part of our Open Feedback Publishing System. The idea is simple: improve in-progress books by engaging the community in a collaborative dialog with the authors out in the open. To do this, we followed the model of the Django Book, Real World Haskell, and Mercurial: The Definitive Guide (among others) and built a system to regularly publish the whole manuscript online as HTML with a comment box under every paragraph, sidebar, figure, and table.