For further reading on all things Celtic:
The Magic Arts of Celtic Britain—Lewis Spence – Fascinating book with index.
Before Scotland – Alistair Moffat
Celtic Scotland – Ian Armit
Scottish Crofters – Susan Parman – Ph.D Thesis. Hard to find. Pub in 1990
Washerwoman at the Ford – Fiona MacLeod (William Sharp)
The World of the Celts – Simon James
Celtic Gods and Heroes – Marie-Louise Sjoestedt
Myths and Legends of the Celtic Race – T. W. Rolleston – Good place to start – no index
Superstitions of the Highlands and Islands of Scotland – J. G. Campbell – pub 1900.
Popular Tales of the West Highlands, Volumes 1 to 4 – J. F. Campbell – First pub 1890.
The Druids – Stuart Piggott
The Mabinogion – Lady Charlotte Guest. Also Sioned Davies (translation).
The Gallic Otherworld – J. G. Campbell – Taken from books first published in 1902
Daily Life of the Pagan Celts – 2009 – a modern approach – has index
Celtic Britain – J Rhys – 1904 – The reference for many of the others – Dense. Has good index
The Fairy-Faith in Celtic Countries – W. Y. Evans-Wentz – Dense. More for sampling than reading
Iron Age Communities in Britain – Barry Cunliffe – Huge book, first pub 1974
Mythology of the British Islands – Charles Squire – a reference for many of the other books, yet very readable. First pub 1905
Celtic Mythology and the religion of the Ancient Celts – Dr Jesse Harasta – short book, lots of pictures.
Sir Gawain and the Green Knight – several
Le Morte d’Arthur books 1 & 2 – Sir Thomas Mallory. The English version of the Irish and Welsh gods.