According to a 2012 survey of college students in Florida, because of the high cost of textbooks:

• 64% did not purchase a required textbook
• 49% took fewer courses
• 45% did not register for a course
• 27% dropped a course
• 21% withdrew from a course

And in a report from the Student PIRGs, "82% of students felt they would do significantly better in a course if the textbook was available free online and buying a hard copy was optional."

If your textbook is online, you can start using it from the first day of class: no need to provide extra time for students to buy a copy!

According to surveys from 2012 and 2014 from the University of Central Florida:

• Lower cost and convenience are the top reasons students purchase an e-textbook, not the interactive features designed to enhance learning.
• Around 40% of participants cite a general preference for print textbooks.

The books that can be found in these directories can be read online for free, and most can also be downloaded in various formats. Most are made available under a Creative Commons license that will allow you to redistribute it and possibly even modify it for your own use. Be sure to check the terms of use for the specific book you are using.

Free textbook alternatives, just like conventionally published textbooks, vary in quality. The directories listed above each have a mechanism for reviewing textbooks to be included.

See Open Education Handbook 2014, a continually updated online book, and a literature review of empirical studies on the effects of use of OERs.