Can a folk tales compilation/book be a valid source of CC0 texts?
See, the compilation author explicitly writes that 1) the book is a collection of folk tales and 2) he is an editor.
It should as long as the author clearly publish them under a valid license or gives his/her consent to re-license it under it.
… but the tales’ author is ugh… folks, like in tales were invented by people long-long time ago and no one knows who are their authors… There was no notion of public domain when these tales were created
My understanding is that the author is whoever compiled the stories and transcribed them into text and published it.
Similar to translations, some classic books are public domain (the original texts) but translations are published maybe a few decades ago by someone and are still under some kind or copyright.
Ok, this makes sense. Just to make things clearer – one of my friends who authored a booklet allowed to use his material. In which form (and to whom) should I show that he has CC0’d his authorship? Does a facebook chat-conversation screenshot work?
I would say that ideally the content is hosted in place people can consult for reference where the author clearly communicates the license, it could be his own blog/site, github or any other platform where you can upload this kind of material. So in case anyone asks we can direct them there for reference.