Chadwick, Nora. “Imbas Forosnai,” Scottish Gaelic Studies 4 (1934):97-135.
Chadwick, Nora. “Dreams in Early European Literature,” James Carney and David Greene eds. Celtic Studies: Essays in Memory of Angus MathesonLondon:Routledge Kegan Paul, 1968. 33-50.
Chadwick, Nora. Poetry and Prophecy Cambridge: Cambridge University Press, 1942. Chadwick was a pioneer in this area of Celtic studies, and is still worth reading if you can find a copy.
Eliade, Mircea. Shamanism: Archaic Techniques of Ecstatsy. trans. Willard Trask. Bollingen Series 76. Princeton: Princeton University Press, 1964. ISBN 0691017794. This is the book to start with; the first serious scholarly study of Siberian shamans. Amazon catalog page for this book.
Eliade, Mircea. Rites and Symbols of Initiation: The Mysteries of Birth and Rebirth. Trans. Willard Trask. New York, 1975. ISBN 0882143581.
Eliade is still a primary scholar in terms of shamanism and ritual. Amazon catalog page for this book.
Ford, Patrick. “The Blind, the Dumb and the Ugly: Aspects of Poets
and Their Craft in Early Ireland and Wales,” Cambridge Medieval
Celtic Studies 9. A super article about the role of the poet in
Celtic literature, and a good companion to Nagy, Wisdom of the
Melia, Daniel F. “Law and the Shaman-Saint” Patrick K. Ford ed. Celtic Folklore and Christianity: Studies in Memory of William W. Heist. Santa Barbara, CA: , 1983.
Nagy, Joseph Falaky. “Liminality and Knowledge in Irish Tradition”
Studia Celtica 16/17 (1981/82): 135-143. I also strongly recommend his Wisdom of the Outlaw. University of California Press: Berkeley, 1985, on the Finn tales.