Fire in the Head: Shamanism and the Celtic Spirit by Tom Cowan is a comparative analysis of Celtic mythos with Shamanism found worldwide.
In the introduction the author establishes the definition of "Shamanism" and "Celts" in order for the reader to best understand how he is handling the topic. The next chapters cover a variety of topics, such as the roles of fairies, music, power objects, heroes, other worlds, and journeys.
Some topics are covered in detail, like the King Arthur stories, but the rest of the topics seem to be mostly a glossing. It seemed to me that more often than not the author ran out of things to say about the Celtic mythology or practices, so he just fell back on making comparisons to other Shaman traditions from around the world.
I have read a lot of books on Shamanism, so much of that material was familiar to me. I only consider myself briefly versed in Celtic mythology, yet I was disappointed with what little material about the tradition was actually contained in the pages. For those who are unfamiliar with both of these topics, this would probably be an excellent read; however, for those who are familiar with both of these topics, this isn't worthwhile.
The Fairy-Faith in Celtic Countries by W. Y. Evans-Wentz
Shamanism: Archaic Techniques of Ecstasy - Mircea Eliade