I actually read this book a long time ago - and then I picked it up again at the beginning of the year to dive into, here and there, like a reference book, to help me with my research on Taliesin who is one of the characters in my upcoming book The Grail Mysteries.
I find that John Matthews has provided the best of the both worlds with this scholarly seminal work on Celtic shamanism. He is a thorough, clear-minded scholar who is guided solely by the evidence and not just some kind of rainbows-'n-unicorns New Age thinking. At the same time, he perceives the ancient works of the poet Taliesin through a shamanic mind-set - which is not New Age and is, in fact, quite Old Age - and he manages to transmit those concepts to the reader in an easy-to-understand form.
I would go further - I think, in this book, Matthews has done more to resurrect Celtic shamanism than any one writer and then to show others the way. I expect to be picking up Taliesin: The Last Celtic Shaman again one day; it's just that sort of book!