There are some open source and free textbooks online that are good background resources for our group. The British Columbia’s BCCampus OpenEd is a good entry point for discovering free digital textbooks written by higher education faculty. For example, the Social Psychology title touches on several topics we often hit on, such as in-/out-group dynamics. Another good source is OpenStax.
If you find other good resources, especially at low or no cost, please share their links.