Over on the Prawfs' Blawg, Eric Johnson has posted a link to his completed 2 volume torts case book, which can be downloaded for free.
Unlike most casebooks, Johnson's Torts Compendium does not contain notes and questions to accompany the cases. It's pure source material. So, it makes sense that his book would be free (to download, you still have to bear the cost of printing it), everything in it is part of the public domain.
The lack of commentary will make the book of limited use. The autodidact who realizes you don't actually have to go to law school to educate yourself about the law won't be provided with some necessary history and context. Students in most law school classes will still have to shell out money for a book with whatever numbers their prof wants written at the bottom of the page. The main beneficiaries will be his own students, and the students of professors who choose to use this book instead of the more expensive options.
But, his book highlights an interesting issue. The vast majority of material in other casebooks is also part of the public domain. The other things which may be copyrighted, such as essays or law review articles, are available to law students for free through their Lexis or Westlaw accounts.
The only things your traditional casebook provides are some notes, which are typically no more informative than the case's Wikipedia article, and questions without any answers or analysis.
Sure, there's also some editing done to cases to leave out the irrelevant parts, and Johnson's book does this as well, but what exactly are law professors charging their students for in case books, and why don't more follow Johnson's lead? You're already getting a salary paid by students' tuition, isn't one trip to the well enough?