OpenStax, part of Rice University, is an educational initiative that releases and updates free (a great word!) textbooks. Subjects include, for example, sociology, economics, astronomy and psychology. The books can be downloaded directly from their website in PDF format. It is also possible to download the textbooks as an e-book from the Kindle store (search 'openstax' to find the books). OpenStax also hosts open educational resources that supplement the textbooks.