An open textbook project developed at Rice University, OpenStax offers open introductory textbooks that can be accessed in different formats. Books are also available in print, for a fee. All OpenStax texts have a CC-BY license, which allows users to remix and redistribute the work, so long as the original work is credited.
The Open Textbook Library at the University of Minnesota hosts nearly 700 open textbooks in a range of disciplines, which can be downloaded in multiple formats. Pay attention to the Creative Commons license of any book you may want to use, as remixes may not be a permitted use.
BC Campus OpenEd offers more than 300 open textbooks. You can search or browse for titles in your discipline. The Creative Commons license differs for each title, so pay attention to what uses are permitted by the license.