Keep in mind that many open textbooks have been peer reviewed by faculty or subject matter experts so you may use these reviews to narrow down choices before examining them yourself. If you want to evaluate the materials yourself, there are some existing rubrics that can guide you.
The BCcampus Open Textbook project has published open textbook review criteria that is used by faculty who provide peer reviews for its open textbooks. The criteria was derived from an earlier rubric created by the College Open Textbooks Collaborative and has also been adopted by the Open Textbook Library.
Achieve, a non-profit organization dedicated to raising graduation rates and getting student college and career-ready, has also published an evaluation rubric for OER. This rubric raises the important issue of the accessibility of learning materials for students with disabilities. You should consult with your Disability Services Office on campus whenever selecting instructional materials to ensure that materials meet these standards.
OpenStax All Subjects
OpenStax Math Textbooks
OpenStax Science Textbooks
OpenStax Social Science Textbooks
OpenStax Humanities Textbooks
OpenStax AP Textbooks
