Open Educational Resources or OERs (including open textbooks) are any content which is:
For links to major collections, subject collections, open courses, textbooks, and more, see NWTC's Open Educational Resources Guide.
A great place to search for Open Educational Resources is through the Creative Commons site.
NWTC also has several custom OER textbooks in the various libraries from LibreTexts. This site makes it easy to find OER content! Contact Kim LaPlante if you would like her to create a custom textbook for you from chapters found on LibreTexts.