• Openstax College: http://openstaxcollege.org/books
• Openstax CNX: http://cnx.org/
• Open Textbook Library: http://open.umn.edu/opentextbooks/
• College Open Textbooks: http://www.collegeopentextbooks.org/
• Saylor Foundation: http://www.saylor.org/books/
• Open Culture: http://www.openculture.com/free_textbooks
• Bookboon.com: http://www.bookboon.com
• The Global Text Project: http://globaltext.terry.uga.edu/books
• BC Campus: https://open.bccampus.ca/find-open-textbooks/
• California Open Library for Education: http://www.cool4ed.org/
• Free Book Centre: http://www.freebookcentre.net/
• InTech: https://www.intechopen.com/
• Open SUNY Textbooks: https://textbooks.opensuny.org/
• Wikibooks: https://en.wikibooks.org/wiki/Main_Page
• Textbook Revolution: http://textbookrevolution.org/index.php/Book:Lists/Subjects