An open textbook is an integrated course-associated learning tool that is in the public domain or has been open-licensed by the copyright holder to permit re-use without the necessity of asking permission of the copyright holder. Open textbooks are free or inexpensive on the web and modestly priced for downloads, use on eReaders, or in bound format.
Some of the platforms shown here host the textbooks and encourage reuse and customisation of the works for specific courses. Others, like the Community College Consortium of OER and The Orange Grove (Florida) act an as aggregator for texts on other sites. The latter has a section on texts suitable for K-12 education.
Most of the textbooks have Creative Commons (CC) open licences or GNU - Free Document Licence. Others are U.S. government documents in the public domain. Many other textbooks are free to view online but are NOT OPEN for reuse and customisation. See Copyrighted Digital Textbooks for a list of learning content without open licences.