Open textbooks are part of a larger movement called "Open Educational Resources" (OER Background)
An open textbook is a textbook with an open copyright license that allows the material to be freely accessed, shared and adapted.
Open licenses allow instructors to adapt, remix, and customize existing open textbooks so that the content meets their own learning objectives.
Many open textbooks are developed through traditional peer review, others are vetted by experts. As with any textbook, the instructor is the final judge of whether an open textbook meets the needs of the course.
They are free for anyone to view and download in an online format and can be made available in a print format at a nominal cost.