J. A. Macculloch 
Religion of the Ancient Celts [PDF ebook] 

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. And we are left to our own interpretations, more or less valid, of the existing materials, and to the light shed on them by the comparative study of religions. As this book was written during a long residence in the Isle of Skye, where the old language of the people still survives, and where the genius loci speaks everywhere of things remote and strange, it may have been easier to attempt to realise the ancient religion there than in a busier or more prosaic place. Yet at every point I have felt how much would have been gained could an old Celt or Druid have revisited his former haunts, and permitted me to question him on a hundred matters which must remain obscure. But this, alas, might not be.