Prydain Chronicles. They are nominally based on the Mabinogion of Welsh mythology. I've wanted to read the Mabinogion for some time, so I picked up the Penguin Classics edition years ago. Just never got around to reading it until now.
And, needing further information for/distraction from writing the book, I started reading.
First off, I will tell you that my good friend Kate lived in Wales during grad school and speaks a bit of Welsh. I visited and can speak no Welsh. I was incredibly happy that this edition has a lovely pronunciation guide for all the Welsh people and place names. Otherwise, I would have been emailing Kate left right and center.
Second, it was such a treat to read a different take on the origins of King Arthur, King Mark, etc. Knowing that the Mabinogion (as it is known now) wasn't collected or written down until after the rise of Christianity in the British Isles puts a different spin on character motivations and actions. It would be so interesting to see what the pagan origins of the myths are but we will likely never know since that religion waned before the tales were recorded.