Since all of our material is governed by the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License (https://creativecommons.org/licenses/by-nc/4.0/), adding traditionally copyrighted materials in your Flexbook® textbook is not permitted without the express permission of that material's copyright holder.
It is your responsibility to ensure content you add to a FlexBook® textbook is available under a license compatible with CC BY-NC, or is your original work. All CK-12 content is licensed in similar ways to allow for sharing and derivations.