Most text books are riddled with errors. So I guess as far as content goes the value of groups will be in correcting such errors.
(And perhaps also in better explaining the material by adding notes and linking to external content.)
(And perhaps also in better explaining the material by adding notes and linking to external content.)
Here's an example of people working through an issue in a C programming class: http://curiousreef.com/class/programming-in-c/lesson/1/assgn.... There wasn't any issue with the actual book itself, but the discussion is adding value to others who might come across the same problems.