"Open Educational Resources" by Ron Mader is licensed under CC BY 2.0
OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. ~ William & Flora Hewlett Foundation
For a more comprehensive overview on OER, see:
"Mixing Console" by Mark Catoe is licensed under CC BY 2.0
This is a sample of several OER textbooks or course materials available for Audio Technology & Industry. Additional OER in your subject may be found by searching the resources listed on the ACC Libraries primary Open Educational Resources Guide.