A selected list of large OER repositories.
OERs are not just textbooks but include all sorts of materials including:
and more that can be remixed and shared with little to no restrictions.
Open education resources depend on a type of licensing that allows the copyright owner to freely distribute their materials under certain terms.
The CC license puts certain restrictions on materials, and there are different permissions associated with each license.
This site, about publishing a book using a CC license, has a pretty good overview of the process, and has a nice charted summary of the symbols and the permissions they allow.
When you are considering using someone else's OER materials for your class, make sure you examine the particular CC license. Most allow re-use with attribustion for non-commerical purposes. However, some licensing does not allow for re-mixing or adaptation.