Sources for textbooks were selected with Creative Commons CC By licensing, allowing you to download, reuse, revise, and redistribute the content.

A curated catalog of open college textbooks hosted by the University of Minnesota; a good place to start your search.
Directory of Open Access BooksThe Directory of Open Access Books offers links to previously published books under Creative Commons licensing.
College Open TextbooksThe College Open Textbooks is a collaborative project of colleges, governmental agencies, education organizations supporting OER textbooks.
The Applied Math & Science Education Repository offers OER textbooks in science and technology as well as links to other resources.
ScholarWorks@GVSUGrand Valley State University offers a small collection of books with Creative Commons licensing.
A collection of UC Press books made Open Access, published 1982-2004.
The OpenStax Catalog features open textbooks; hosted by Rice University and its OpenStax CNX platform.
A list of books CalTech faculty have published and offer as Open Access.
Use the Advanced Search in Google Books to narrow to Full View and subject.
Another search interface for Open textbooks and courses.
Mathematics textbooks for college level courses.
University and academic press books that have been published as Open Access texts from European publishers.
British Columbia Ministry of Advanced Education, Innovation, and Technology supported OER textbook effort.
A growing collection of Open textbooks from a SUNY Press and library collaboration. High quality texts for a range of courses.