After reading this book, I came to realize that the approach the John and Caitlin Matthews is using expresses the spirit of Celtic Spirituality as it presented in sources such as Celtic Heritage by Alwyn and Brinley Rees and The Gods of the Celts by Miranda Green. However, It would have been nice if John and Caitlin Matthews would have used these text as sources for their work. Much of the text expresses what is apparent in ancient Celtic religion while other aspects are off into left field. The only reason for this that I can think of is that the translations that are being used are out dated (some seem to be from the 1900s) as well as looking at late middle ages sources and saying that they come from ancient Celtic sources, which would be impossible to tell. Even with all the problems that arise with the text, it is a wonderful approach and much of what John and Caitlin Matthews express, from what I can tell, is consistent with other academic text about Celtic spirituality and religion.
The Encyclopaedia of Celtic Wisdom is, hands down, required reading for anyone interested in the primal Celtic traditions, especially of the Irish and Scottish traditions. Where John Matthews' Taliesin and the Shamanic Mysteries of Britian addresses largely a Cymric (Welsh) and British leaning, The Encyclopaedia of Celtic Wisdom is, largely, rooted in the Irish, with some exploration of folk Scots beliefs and expressions of the shamanic (as in the case of the late and well-loved Scottish seer, Eliadh Watt). As of January 2002 I consider this book to be one of the top three books on Celtic tradition, especially the visionary tradition of primal Celtic spirituality and the field of contemporary Celtic spirituality. It is a priceless addition to one's library, along with Tom Cowan's Fire in the Head: Shamanism and the Celtic Spirit, Mara Freeman's Kindling the Celtic Spirit. For a more classical exploration of these same themes I recommend The Silver Bough, by F. Marian McNeill.