Below is a list of resources I would recommend on the Brythonic tradition.

Medieval Welsh Literature

A.O.H. Jarman (transl.), Aneirin – Gododdin, (Gomer Press, 1998)
Marged Haycock (transl), Legendary Poems from the Book of Taliesin, (CMCS, 2007)
Marged Haycock (transl), Prophecies from the Book of Taliesin, (CMCS, 2013)
Meirion Pennar (transl.), The Black Book of Carmarthen, (Llanerch Enterprises, 1989)
Mererid Hopwood, Singing in Chains, (Gomer Press, 2004)
Patrick Ford (transl), Mabinogi and Other Welsh Tales, (University of California Press, 2008)
Peter Bartrum, A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000, (National Library of Wales, 1993)
Rachel Bromwich (ed), The Triads of the Island of Britain, (University of Wales Press, 2014)
Rachel Bromwich and Simon D. Evans (eds), Culhwch and Olwen, (University of Wales, 1998)
Sioned Davies (transl.), The Mabinogion, (Oxford University Press, 2007)
Will Parker, The Four Branches of the Mabinogi, (Bardic Press, 2005)

Brythonic Gods and Goddesses

Caitlin Matthews, Mabon and the Guardians of Celtic Britain, (Inner Traditions, 2002)
Caroline Wise, Finding Elen: The Quest for Elen of the Ways, (Create Space, 2015)
Lee Davies (ed), The Grey Mare on the Hill, (Grey Mare Books, 2015)
Miranda Green, The Gods of the Celts, (Bramley Books, 1986)
Robin Herne, Old Gods, New Druids, (O Books, 2009)
Sheena McGrath, Brigantia: Goddess of the North, (Lulu, 2016)
Susie Fox, Ritual Journeys with Great British Goddesses, (Trafford, 2012)
Tim Clarkson, Scotland’s Merlin, (Berlinn, 2016)

Celtic Culture

Anne Ross, Pagan Celtic Britain, (Cardinal, 1974)
Bernard Maier, Dictionary of Celtic Religion and Culture, (The Boydell Press, 1997)
Barry Cunliffe, The Ancient Celts (Penguin, 1999)
Barry Cunliffe, Britain Begins, (Oxford University Press, 2013)
Francis Pryor, Britain BC, (Harper Perennial, 2004)
John T. Koch (ed), The Celtic Heroic Age, (Celtic Studies Publications, 2003)
Miranda Aldhouse Green, Sacred Britannia, (Thames and Hudson, 2018)
Miranda Green, Animals in Celtic Life and Myth, (Routledge, 1998)
Miranda Green, Dying for the Gods, (The History Press, 2002)

Folk and Fairylore

Angelika Rudiger, ‘Y Tylwyth Teg. An Analysis of a Literary Motif,’ (Bangor University, 2021)
Edward A. Armstrong, The Folklore of Birds, (Dover Publications, 1958)
Francis Jones, The Holy Wells of Wales (Cardiff, 1992)
Katherine Briggs, The Fairies in Tradition and Literature, (Routledge, 1967)
John Rhŷs, Celtic Folklore: Welsh and Manx (Oxford, 1901)
T. Gwynn Jones, Welsh Folklore and Folk-Custom, (Redwood Burn, 1979)
Wirt Sikes, British Goblins, (Lightning Source, 1880)
W. Jenkyn Thomas, The Welsh Fairy Book, (Dover Publications, 2001)
W. Y. Evans Wentz, The Fairy Faith in Celtic Countries, (Colin Smythe Limited, 1977)

The Old North

Guy Ragland Philips, Brigantia: A Mysteriography, (Routledge, 1976)
Nick Higham, The Northern Counties to AD1000, (Longman Group 1986)
Nicki Howarth, Cartimandua: Queen of the Brigantes, (The History Press, 2011)
Tim Clarkson, The Men of the North: The Britons of Southern Scotland, (John Donald, 2010)
Ronald Cunliffe Shawe, The Men of the North, (Leyland Printing Co., 1970)

Celtic Resources Online

Celtic Literature Collective
Celtic Source
Mabinogion.info
Mabinogi.net
Sacred Texts

Awenyddion and Celtic Polytheists

Awen ac Awenydd
Dun Brython
From the Forests of Arduinna
Grey Bear in the Middle
Go Deeper
Greg Hill
Land Sea Sky Travel
Living Liminally
Musings of a Scottish Hearth Druid
Rigantona
Starstruck Awenydd
The Druid’s Well

Polytheistic Monasticism

Afallach Emrys, The Path of the Sacred Hermit: Exploring Monasticism in Modern Pagan Spirituality (Barnes & Noble, 2023)
Janet Munin (ed), Polytheist Monasticism: Voices from the Pagan Cloisters, (Moon Books, 2022)
Oisin Doyle, A New Monastic Way: Modern Polytheistic Celtic Monasticism, (Fellowship of the Nine Waves, 2017)

Pagan Monastery Podcast
The Black Stone Sanctuary
The Cloister
The Gnostic Celtic Church Monastery
The Order of the Sacred Nemeton