You can openly licence your Open Textbook by attributing your work using a Creative Commons (CC) licence. A Creative Commons licence is one of several public copyright licences that enable the free distribution of an otherwise copyrighted "work". A CC licence is used when an author wants to give others the right to share, use, and build upon their created work.
The OER (Open Educational Resources) guide will provide instruction on how to openly attribute your work. The best practice is to maintain a record of permissions for the use of third-party copyright content.
Your School Librarian can provide further advice and assistance with Creative Commons (CC) licencing and attributions.