Here are some key sources for finding open textbooks but also open books. Some websites serve as discovery hubs, searching across multiple repositories that include textbooks and supplementary materials. Others act as host repositories, offering curated texts with academic reviews.
This page has Open Access Resources as well as Open Educational Resources.
A community-driven discovery service that indexes and provides access to trustworthy, scholarly, peer-reviewed open access books.
Textbooks that are free to use and distribute, and licensed to be freely adapted or changed (with proper attribution).
Pressbooks Directory is a free, searchable catalog that includes 8,196 open access books published by 199 organizations and networks using Pressbooks.
Haithi Trust preserves 17+ million digitised items - the largest set of digitised books managed by academic and research libraries under the aims of scholarly, not corporate, interests.
The OAPEN Library contains freely accessible academic books, mainly in the area of humanities and social sciences.
Free college text books (US based, so the content reflects that).
Provides access to over 60,000 free eBooks; also discoverable through UR Library Search.