Here are some useful resources.
Breatnach, Liam. “The Caldron of Poesy”. Eriu 32. 1981.
Calder, George. Auraicept na n-Eces: the Scholar’s Primer.https://archive.org/details/auraiceptnancess00calder
Carey, John. “The Waters of Vision and the Gods of Skill”. Art and the Sacred Kairos and the Dallas Institute for Humanities and Culture. 23 March, 1991. Santa Fe.
Carney, James. Medieval Irish Lyrics with The Irish Bardic Poet.
Chadwick, Nora. Poetry and Prophecy. Good worldwide survey form 1952 (but does contain some racist/colonialist attitudes prevalent in that era).
—, Chadwick. “Imbas forosnai”. Scottish Gaelic Studies, 1935.
Corkery, Daniel. The Hidden Ireland.
Ford, Patrick. “The Blind, the Dumb, and the Ugly”. Cambridge Medieval Celtic Studies 19.
Guyonvarc’h, Christian J. The Making of a Druid: The Hidden Teachings from The Colloquy of Two Sages (the text with annotations).
Heaney, Seamus. Sweeney Astray: A Version from the Irish. Irish poet’s translation of Suibhne Geilt, the poetry of “mad Sweeney’, a glimpse into the world of a geilt, outsider poet living in the woods.
Henry, P. L. “The Caldron of Poesy”. Studia Celtica 14/15. 1979/80.
Jones, Mary. Jones Celtic Encyclopedia. 1998-2015. Web.
Laurie, Erynn Rowan. The Well of Five Streams (contains her Cauldron of Poesy article).
Minahane, John. The Christian Druids: on the filid or philosopher-poets of Ireland.
Nagy, Nagy, Joseph Falaky. The Wisdom of the Outlaw: The Boyhood Deeds of Finn in Gaelic Narrative Tradition. Berkeley and Los Angeles: U of California Press, 1985. The Fenian outsider warriors were poets too.
Ó hÓgáin, Dáithí The Sacred Isle. A crucial book for understanding pre-Christian practices and believes in Ireland by an Irish Celticist.
Ó hÓgáin, Dáithí. Myth, Legend and Romance: An Encyclopaedia of Irish Folk Tradition. London: Ryan, 1990.
Ó Tuathail, Sean. The Excellence of Ancient Word: Druid Rhetorics from Ancient Irish Tales. Idiosyncratic modern practitioner’s take is worth a read.
Patterson, Nerys. Cattle Lords and Clansmen: The Social Structure of Early Ireland. The historical background.
Skelton, Robin. Samhain and other poems in Irish Metres of the Eighth to the Sixteenth Centuries. Contains an appendix with the different traditional meters.
Thompson, Christopher Scott. A God Who Makes Fire: the Bardic Mysticism of Amergin. A recommended practitioner’s handbook.