There are not many books about Gaelic and Brythonic Traditional Witchcraft. However, there are a few resources. Again, I am only listing books that are reasonably priced. This book rec list is not Celtic Recon, recons will not nec like any of these books.* This is for witchery.

Fairy Faith in the Celtic Countries by WY Evans-Wentz

Irish Witchcraft for the Irish Witch by Lora O’Brien

A Cornish Book of Ways and the upcoming Black Toad by Gemma Gary

Ogham Weaving Word Wisdom and Circle of Stones by Erynn Rowan Laurie, not about witchery, but magics a witch can certainly utilize.

The Ballad of Biddy Early by Nancy Willard, this is a childrens picture book, get it anyways alternately if you are above picture books, Biddy Early Wise woman of Clare by Meda Ryan

The Guises of the Morrigan by Rankine and D’Estes—read the legends of this witch goddess for examples of curses, glamours, shape shifting, and prophecy. If you like that one get their Callieach book too. (Both appear on my “free today” notices fairly often, just follow me ;)

Brida a novel by Paulo Coelho. Sometimes fiction can be true. I like this one. One of the characters is named Wicca, but she is not Wiccan its just a little idiocyncracry, try and get past it.

Witchcraft and the Second Sight in the Highlands and Islands of Scotland by Campbell

The White Goddess by Robert Graves, now you aren’t reading this for truefax ancient myths, but for witchery, so look at it differently, Graves was a witch, not a historian.

Bonus: Imramma website by Eblanna Raven http://eblannaraven.com/ and the ITW website which has excellent magic theory articles http://www.traditionalirishwitch.com/ 

*There is the CR FAQ for people who are interested in CR, check out their book list. Again my above list is not a CR list—its for Traditional Witchcraft—which is not even necessarily polytheist, any pagan resources are listed as inspiration for witchcraft or for its hints at mysteries.