Open Educational Resources consist of books, articles, images, videos, curricula, and other materials which are free to be used in educational settings, subject to certain limits, without payment. These resources exist either in the Public Domain, or under one of several types of Creative Commons license.
You can see a selection of definitions of OER from Creative Commons here.
When using OER websites bear in mind that open means open; some sites are actually commercial enterprises.
The library can offer access to textbooks which are not defined as open since the library must pay for them, but which are free to students.
The Faculty Select portal is a good starting point for identifying this type of textbook, but faculty may discover some of their own sites. The library will in most cases buy any unlimited user ebooks an instructor may request for use in courses.