I am working on an open-access textbook. I have the permission of an author to use material from a previously published book from 20 years ago which the publisher returned the rights to digital publication. Is there a best practice for establishing that something has been released under an open-access license? I am using excerpts from the book and citing all of them and indicating the CC-BY-etc status we've agreed upon by the authors. They are near retirement and their website could someday disappear so I am putting up PDF copies of their original book on a data repository (Open Science Framework) and adding citation information and CC-BY-... info.
UPDATE: Some relevant additional information
- I'm currently integrating the material into a CC-BY project and using the material based on an agreement via email. The authors and I want to make a definitive release of the material under the license we discussed, separate from its appearance in my project.
- The original authors are emeritus/near retirement, so I'm looking for a way to establish that the materials are CC-BY-... for myself and the broader community
- Related to the above point, the web presence of the original authors may decay/go away when they fully retire