Open Textbooks are an important Open Educational Resource that faculty and students are free to Use, Share, Adapt and Remix. They have many advantages:
Open Books are similar to Open Textbooks in that they are free to Use and Share on sites like Moodle - but unlike Open Textbooks their Creative Commons Licenses generally do not include Adapting or Remixing in them. This makes them more like Open Access journal articles. So you need to check an Open Books license to learn if adapting or remixing is allowed.
This guide includes both as they are both useful for online courses. I am also including locating and linking to library licensed (not free) content as a means of ensuring you are made aware of all possible online course materials.
Affordable Learning Georgia has a good document on evaluating an Open Textbook. It includes information on Open Textbook Quality Criteria and Ancillary Materials Quality Criteria. It is useful and too the point.