The Mabinogion is one of my favorite books of all time.I like all Celtic mythology, but the most interesting to me are the Irish and the Welsh myths. Regarding the latter, I recommend the myths of "Culhwch and Olwen" and "The Dream of Rhonabwy". I stumbled upon them when researching for a project at the library, and I liked them so much that I looked for more books on Welsh mythology (which, in my opinion, is probably more fascinating than the Greek one).