I am more than a little wary of the Matthews. Also check out--The Druids, by Stuart Piggot; Pagan Celti Religions of the Ancient British Isles, by Ronald Hutton; Life and Death of a Druid Prince, by Anne Ross, and The Celts, by Nor Chadwick (slightly dated). T.G.E. Powell is also OK, if also slightly dated also (1958)