How do you release a previously published, copyrighted work as open access?

Question

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

1. 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.
2. 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
3. Related to the above point, the web presence of the original authors may decay/go away when they fully retire

Answer 1

If all you mean is how do you make it clear that it is proper to republish the work, then just put a notice at the beginning of it to that effect, naming the original publisher and noting that rights to digital publication were returned to the authors.

Probably, but not necessarily, the original publisher still holds the fundamental copyright so you should also make that clear. Or make clear what the copyright status is.

But a statement in the work itself is the way to "advertise" the facts. Keep any paperwork in case you are ever questioned about it.