Brogan, T.V.F, ed., The Princeton Handbook of Multicultural Poetries, Princeton University Press, Princeton 1996 -- an excellent encyclopedia of global poetic traditions, including an extensive article on Irish poetry.

Chadwick, Nora K., "Imbas Forosnai," Scottish Gaelic Studies vol 4 pt 2, 1935 -- this seminal article discusses some of the trancework divinatory techniques of the early filidh. It is a comlicated article, but one well worth reading, and can be found on the net here

Chadwick, N. Kershaw, Poetry & Prophecy, Cambridge University Press, Cambridge 1942 -- a wide-ranging examination of the practice of prophetic poetry around the world

Degler, Teri, The Fiery Muse: Creativity and the Spiritual Quest, Random House of Canada, Toronto 1996 -- this book explores the connections between creative genius and mystic experience, which is the juncture where true imbas lies. Difficult to find, but well worth reading.

Hirschfield, Jane, Nine Gates: Entering the Mind of Poetry, HarperCollins, NY 1997 -- an excellent and lyrical examination of a number of different issues in poetry and poetics, from concentration to translation. Exquisite and highly recommended.

Lord, Albert B., The Singer of Tales, Harvard University Press, Cambridge 1960 -- techniques of oral poetry and epic

Mercier, Vivian, The Irish Comic Tradition, Oxford University Press, Oxford 1962 -- contains an excellent introduction to the traditional practice of satire, an early Irish magical tradition of cursing and blasting

Murphy, Gerard, Early Irish Lyrics, Clarendon Press, Oxford 1956 -- essential examination of Irish metrics containing poetry in Gaelic and English translation

Nagy, Joseph Falaky, The Wisdom of the Outlaw: The Boyhood Deeds of Fionn in the Gaelic Narrative Tradition, University of California Press, Berkeley 1985 -- excellent discussion of Fionn as fili and the function of the fili as outsider

Rao, Velcheru Naryana and David Shulman, A Poem at the Right Moment: Remember Verses from Premodern South India, University of California Press, Berkeley 1998 -- an excellent overview of Indian oral poetry, a tradition that has many startling parallels to the filidh of early Ireland. It also examines the role of Sarasvati, whose place in Indian poetry is extremely similar to that of Brighid in Irish and Scottish poetic tradition. Extremely relevant and important, not to mention being filled with lovely poetry.

Skelton, Robin, Spellcraft, McClelland and Stewart Ltd., Toronto 1978 -- an excellent guide to poetry as magic, possibly the only book on the subject -- this book has apparently been reprinted by a different publisher

Skelton, Robin, Samhain and other poems in Irish Metres of the Eighth to the Sixteenth Centuries, Salmon Poetry Press, Dublin 1994 -- good explanations of the metrics of early Irish poetry, with English-language examples. Hard to find but worth it.

Travis, James, Early Celtic Versecraft: origin, development, diffusion, Cornell University Press, Ithaca 1973 -- a look at the history and development of formal verse in Ireland

Turco, Lewis, The New Book of Forms: A Handbook of Poetics, University Press of New England, Hanover 1986 -- according to Skelton, his metrics are incorrect in places, but he does explain all the technical terms and techniques of poetry

Watkins, Calvert, How to Kill a Dragon: Aspects of Indo-European Poetics, Oxford University Press, Oxford 1995 -- an in depth discussion of many aspects and forms of Indo-European poetry and poetics from Iceland to India