The Mabinogion is a work containing medieval Welsh stories. There are four particular stories that are the core of it, but they are generally grouped with several other Welsh stories such a few Arthurian tales. There are several translations (some with variant spellings of the title) available; my copy is the most "user-friendly" translation by Patrick K. Ford (see http://makeashorterlink.com/?R2A052AB6).

As an aside, Fleetwood mac fans may be interested to note that "Rhiannon" is the name of one of the characters in these stories, a woman falsely accused of murdering her son.