Sioned Davies' new English translation of what is popularly knows as The Mabinogion, is a classic collection of Welsh myths and legends originally found in medieval manuscripts. This collection consists of eleven stories featuring Kings, knights, magic, shape-shifting, giants, talking animals, gods...
I have tried reading various translations of The Mabinogion, by far this is the easiest translation to read. It helps that the hardcover has two bookmarks, so you can read the notes easily as you go.As an introduction to these myths, this is a great place to start.