Google Books is one way to get to Google's vast library of e-books (you can also reach it through standard Google Search or through the Google Play store). Here, you can buy and download e-books, and there is a huge trove of free books available as well. You can also view a preview of any non-free book (the size of the preview depends on the publisher).
You can search within the full text of books. This is really useful for students when they are looking for a particular quote. It's also useful for teachers, such as when they want to look up a quote from a student's paper. This search function works with paid books as well as free, even if the publisher doesn't allow previewing (copyright law requires that they allow up to 3 line "snippets" to be available for previewing even without permission, so searches under 3 lines work).
Obviously, if you're a teacher then textbook adoption is probably out of your hands. If not, however, Google Books has a growing selection of college textbooks.
I require my Dual Credit kids to do reading outside of the normal class requirements (for pleasure, basically, only for a grade). I can add the books on the approved reading list to a Classroom Bookshelf and then share that bookshelf with the class.