The Welsh Fairy Book by W. Jenkyn Thomas with illustrations by Willy Pogány (1908).
Available FREE texts:
Sacred Texts Archive
Other online sources:
Kindle eBook: $2.99
If you are curious about the various sources that Thomas used to compile the collection, you can consult the Preface; he lists many different sources, and emphasizes that "nothing has been inserted that is not genuinely traditionary." If you are curious about Wales and its cultural traditions, you can learn more at Wikipedia: Wales.