I picked this up when I was taking a class 'The Celtic World'. The class was fascinating; debunking many of the myths and long held beliefs about where the Celts came from and where they ended up. The professor's particular bugbear was the so-called ancient Celtic spiritual rituals - rituals which have been created and/or developed in the modern world by people searching for a spiritual connection to nature.
This book is subtitled 'A Guide to Celtic Spirituality and Wisdom'. It is very silly. There are tidbits of actual history and actual legend to be gleaned if you dig hard enough, but - the bibliography is minimal and consists mostly of New Age Hipsters blathering on about distorted legends. In fact, of the books listed, 9 are written by the author herself. To be fair, she has included one or two historical documents, like Monmouth's History, but if she did read them, she took them as gospel, rather than fairy tale or mythical legend.
Don't bother with this. ( )