E-books can also be used as textbook alternative or supplemental materials for your course.
OASIS Search: 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 117 different sources and contains 388,707 records.
Mason OER Metafinder-a one-stop searchbox for Open and Affordable Educational Resources
Many of the collections will have links to the same books, but each repository has a particular focus, and items you can't find in other collections.
OER repositories contain more than just open textbooks.