An open textbook is an openly-licensed textbook offered online by its author(s). The open license sets open textbooks apart from traditional textbooks by allowing users to read online, download, or print the book at no additional cost.
The repositories below host numerous open textbooks:
The Mason OER Metafinder (below) searches across 18 different sources of open educational materials.
Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 88 different sources and contains 352,755 records.