In my book (I'm not the real owner, belongs to my son) you can read:
"Adapted by XXXXX from a folk tale / popular story"
How to enter the authorship of the book?
Author - Folklore
Authror - Traditional
Author - Popular
Maybe do not add author.
In the future we should have option to add main artists even if he's not an author, like compiler, or compilations of novels edited by...
There's an Adapted by credit role as well as a Creator credit role. Maybe you could use one of those.