Interested in assigning (or authoring) an open access textbook for your courses? James Grimmelmann at Cornell Law School maintains a list of open access and affordable casebooks. There is also a growing selection of open educational resources available in the OER Commons for graduate-level law courses.
You can learn more about open access casebooks and creating your own at H2O, a "free platform for making, sharing, and remixing open-licensed casebooks and other course materials," developed through a partnership of the Harvard Library Innovation Lab and Harvard Law School Library.