The Religion of the Ancient Celts (Dodo Press)

Comprehensive study from 1911. "The scientific study of ancient Celtic religion is a thing of recent growth. As a result of the paucity of materials for such a study, earlier writers indulged in the wildest speculative flights and connected the religion with the distant East, or saw in it the remains of a monotheistic faith or a series of esoteric doctrines veiled under polytheistic cults."