The Religion of the Ancient Celts by J. A. MacCulloch
There is very little documentary evidence to go on. In particular, we have no actual sacred texts of the ancient Celts, as their texts were transmitted orally only to initiates, and disappeared forever when the last Druid died. Christianity became the dominant religion in the Celtic area before the oral traditions could become written down, unlike the Vedas in India. Ancient Celtic religious beliefs must therefore be inferred from second-hand classical accounts, hints from Celtic mythology, legend and folklore, as well as archaeological and comparative anthropological evidence. MacCulloch marshals this body of evidence, extensively footnoted, so that an authoritative and clear view of ancient Celtic religion emerges.
MacCullough details the Celtic belief in reincarnation and a spectral other world; documents the enormous pantheon of now-obscure gods and goddesses, including many local deities; and describes totemistic and animistic beliefs. In addition, MacCulloch does not flinch (nor sensationalize) when describing the darker side of Celtic practices, including the famous ‘Burning Man’ human sacrifices, cannibalism and exogamous incest.