OER textbooks are often seen as variable.
The Noba Project is designed for Psychology students but allows a number of key features:
One of the most recognized textbook search sites is no more - http://www.collegeopentextbooks.org/
Our valued colleagues at the OU have a selection of resources which incorporate e-textbook style material
BCCampus have an excellent resource in creating electronic textbooks which is more about the principles than an actual resource, but is a textbook in how to create textbooks - very meta!
https://opentextbc.ca/teachinginadigitalage/chapter/oer/
Openstax (openstaxcollege.org), The Saylor Foundation (saylor.org), and Washington State’s Open Course Library (opencourselibrary.org). The Minnesota Open Textbook Library (open.umn.edu/opentextbooks/) provides a clearinghouse of open textbooks and includes faculty reviews of these materials.
The site below is a good example of OER textbooks - as many examples have it, Sciences are well represented, humanities less so