If the author intends for the work to be copyleft, non-commercial, or a combination of those, there are specific licenses within the Creative Commons family that satisfy those requirements. These are already widely used for open-source books on GitHub.

This is why I suggested "any Creative Commons licenses" rather than a specific one. My goal wasn't to spark an argument over which one is the "best", but rather to provide the author with options to choose from, depending on their specific needs.