Red Shelf is an online platform where you can purchase or even RENT e-textbooks.
Vital Source is a library of e-textbooks that you can purchase or rent titles from and access anywhere with capabilities for read aloud – notes and highlights – flashcard and review – and wikipedia lookup.
Kindle Cloud Reader — even if you do not own a kindle you can still purchase e-books through Amazon in the kindle format (oftentimes cheaper than the print version) and access the books through the Cloud Reader.
The Internet Archive — A non-profit library of free books, movies, software, music, websites, etc… This Library offers electronic access to books, among other things. If you create a free account you can check out books to access online. Here is an example of what is available.
Campus Books — buy, rent or sell print books
Chegg Books — buy, rent or sell print books
eCampus — buy, rent or sell print books, with an etextbook option
Books Run — buy, rent or sell print books, with an etextbook option