Check out potential open textbooks for your courses in OER by Discipline Resource Guide: Concordia University.
Open Educational Resources
Open Educational Resources (OER) are teaching, learning, and research materials in any medium that permit no-cost access, use, adaptation, and/or redistribution by others. These open and reusable resources may include textbooks, curricula, syllabi, lecture notes, question banks, audio-video material, assignments, rubrics, etc.
Adapted from "Open Education: Content" (EDUCAUSE)
A revised OER guide will be replacing this version.