Open educational resources (OER's) are teaching and learning resources with an open licence or in the public domain.
OERs can be freely reused, modified, copied or shared.
OERs include Open Textbooks.
Choosing an open textbook for your course means:
OpenStax: high-quality, peer-reviewed, openly licensed college textbooks published by Rice University.
OASIS: openly Available Sources Integrated Search: a search tool that aims to make the discovery of open resources easier.
MERLOT: curated open educational resources, including textbooks.