Trioedd Ynys Prydein provides the text of oldest written Welsh literature and translates it into English with footnotes that provide important input on what the more obscure passages are about. The Welsh triads probably originated as mneumonics devices used to accurately pass down oral tradition by the druids. As the name implies, these stansas are written in groups of three. You can find some of the earliest literature on King Arthur in these as well as recounting historic events and how places came by their name.
I highly recommend this book for anyone interested in the oral traditions of Wales and the "Isle of Britain."