This list is a starting point. These are a few places where you can search for openly licensed textbooks. Some of these repositories contain only textbooks, while others contain other learning materials too.
Access some textbooks from Open Textbook Library in the HACC Library catalog here.
OASIS OER search tool
Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS, developed by SUNY, currently searches open content from 61 different major OER repositories and includes textbooks, courses, audiobooks, videos, books, and more.