Counterexample: What about a reader? These are common both in literature and the social sciences, but the best example is a collection of short stories.<p>In a reader, the editor must pay each one of these authors something. In a juvinile reader, count on 30 stories. (source: <a href="http://activities.macmillanmh.com/reading/treasures/stories/storylist/41.html" rel="nofollow">http://activities.macmillanmh.com/reading/treasures/stories/...</a> as an example.) On top of that, there are questions, extra resources, and accompanying teacher material to write. And you're going to be able to pay for all this in 7 bucks?<p>Perhaps if we limit this to single/dual author textbooks, we can start talking about it. It could be an interesting disruptive system when it comes to tenure, for sure. (Without the curated system, academia will have to adapt.)