Rachel Bromwich and Simon Evans have recently re-edited Culwch ac Olswen, this time including the text in Welsh and the notes in English, and a glossary.

If you are at all serious about Welsh mythology, you will want Rachel Bromwich’s Triodd Ynys Prydein. Bromwich has a super introduction, all the triads, in English and Welsh, and extensive notes on names. Her notes alone are worth the money.