Open textbooks are materials that are freely available, usually through a Creative Commons license, to download, modify, and print in various formats.
Why should faculty adopt an open textbook?
Open textbooks are more affordable than commercially available textbooks and enable student education budgets to stretch further, giving students greater flexibility in their education choices. Faculty can readily customize open textbooks to better meet their local teaching and learning needs. Lastly, open textbooks provide pricing, flexibility and customization advantages that commercially available textbooks currently do not provide.
(from Lansing CC Library)