Open textbook authors are no different than commercial textbook authors, with one exception. Open textbook authors share their knowledge with students freely in order to increase accessibility, and significantly reduce cost to the student.
“Open textbook projects rely on volunteers who are committed to a vision of providing high-quality, freely available textbooks to the worldwide community of educators and learners. Most of the projects mentioned in this module have information on their site on how you can participate in their project.” (from OER Commons Tutorial)
Some open textbooks were originally commercial books whose copyright has run out or whose copyright was returned to the author who has put it under an open copyright. Some authors were hired by a school or foundation to write open textbooks for some particular reason or group of students. Any author writing an open textbook should consult Creative Commons. Most authors use their attribution only or attribution-share-alike license.