No matter where you are in your exploration of open textbooks, there are resources to assist.
Underpinning open is the idea of unhindered access. Traditionally, access to information has gone hand-in-hand with copyright. Open education and practices add a layer to existing copyright that facilitates the sharing on information. It is you choice whether you want a backgrounder on copyright in general or want to dive straight into Creative Commons, the organization established to free information. From there, you might be interested in a deeper exploration of Creative Commons licensing options.
With your appetite whetted, you may be ready for a more fulsome investigation into adapting or creating an open resource. A few favourite titles follow to guide you through the process of opening up your course content.