Sir John Rhys was a Welsh scholar, fellow of the British Academy, Celticist and the first Professor of Celtic at Oxford University.
In 1874 Rhys delivered a series of lectures in Aberystwyth, later published as Lectures on Welsh Philology, which served to establish his reputation as a leading scholar of the Celtic language. This reputation saw him appointed as the first Professor of Celtic at Oxford University in 1877. He was also made a Fellow of Jesus College, Oxford. Rhys was elected bursar of the college in 1885, a position he held until 1895, when he succeeded Daniel Harper as principal.
This is a two volume set originally published in 1901. Covers folklore, beliefs, customs, superstions of people at that time in rural Wales and the Isle of Mann. This is probably 80% or more dealing with the Welsh and covers things like beliefs in fairies, spirits living in wells, trees and lakes and the stories surrounding them, plus lots more. Rhys also goes into the racial aspects of folklore and myth. One thing that made this book great is Rhys actually went to the most isolated places where these beliefs were part of everyday life and interviewed and talked with people compiling first hand reports.
I'm a big believer that folklore and folk customs in the British Isles are a vital source for understanding the Pagan religions of the British Isles because I think much of this stuff is a direct surviving part of Heathenry. These two volumes are one of, if not the, best source for Welsh folk customs.