OpenStax College is an initiative whose purpose is to create and distribute free and low-cost college textbooks. The initiative is led by Rice University. Currently, on OpenStax College students can find six free textbooks on the subjects of biology, statistics, sociology, and physics. The books can be downloaded as PDF and ePub files, viewed online, or downloaded through Apple's iBookstore (the iBook version is not free).
Six textbooks are currently available through OpenStax College and more are in development. For now the books are covering topics at an introductory level.