If you read Hutton's book 'the pagan religions of the ancient british isles ' he gives an excellent description of how much of the 'goddess/celtic' stuff most people quote as fact was utterly fabricated in the 20th century from a muddle of documents (some of them made up in the past anyway).
Also there's that Hobsbawm book 'the invention of tradition' which explains how people reinvent customs to keep their cultures alive