They are often compellingly retold, and more often beautifully illustrated. They are being used for everything from reading aloud to storytelling to creating bridges to other cultures to supplementing curriculums. Readers, both adult and youth, have an inkling that these stories did not spring fully grown from the head of the reteller, they came from someplace. Tell me where. Ask a librarian.
Chances are they'll be able to find a written source.
Then include a specific source note. It would also be nice to include a teensy bit about how the story has evolved. Of any kind. And just for the record, if there is a genre of book that I don't like, I give those books to someone else to review. Clueless picture books.