Open access texts are free for students to read online, usually also free to download, and sometimes also available in inexpensive print format. Remember that open access is not the same as OER and may have more restrictive copyright permissions. Often an open access text may have a Creative Commons license that would allow an instructor to alter the text for their course. These can be supplemented with additional OER resources. This list was compiled by Eastern Michigan State University.
When older out-of-copyright books are assigned, you may be able to find free full text online. An internet search for the title in quotes with the words full text may find it [example Google Search: "Ragged Dick" "full text"]
You may find multiple copies of a work on these sites. A good strategy might be to provide links to more than one source of the book. This lets students choose which version they like best--some are simple text and some have a "turn-the-page" feel, etc.
Below are links to sites that offer full text of classic books. These are not included in the Google Custom Search.
It's a good idea to also stock print copies at the book store for students who prefer print. Some high quality editions of classics might be found with university presses (see Scholarly Presses page of this guide). The following publishers produce inexpensive print versions of classics: