There are a lot of open textbook repositories out there, but I like recommending the ones that have review information clearly available:
OER doesn't always mean replacing an entire textbook or revamping an entire course. OER can also apply to openly licensed materials that can be used as supplementary course material. Here are a links to several collections of openly license materials.
NOTE: Some of the materials might carry stipulations or special requirements to use the materials - see our Open License Options page for more information or contact any librarian for help navigating the many shades of open licenses.