Knowing where people are coming from and where they get their information is always important.  Below are the sources that I have used with my research for my practices.  It is not a conclusive list in any way regarding sources on Indo-European or Gaulish polytheism, but it is a good place to start and I highly recommend all of these sources.

Gaulish Studies:

Ancient Fire: An Introduction to Gaulish Celtic Polytheism by Segomâros Widugeni: This is a go-to for anyone who is looking for a good introduction book to Gaulish polytheism.

The Celtic Gauls: Gods, Rites, and Sanctuaries by Jean-Louis Brunaux: This book is a short, but condensed, overview of the Gaulish Celts and their culture.  It is great for beginners.

Celts and the Classical World by David Rankin: This book is an in-depth look into the classical writers who wrote about the Celts on the continent and their involvement with the Mediterranean world.  The book looks at what was written, who wrote it, where they come from, why they were writing about the Celts, and what other personal biases they had all within a historical context.  The book’s main focus is with the Continental Celts, but it also has a couple chapters on the Iberian and Galatians Celts, as well as references to the Britton and Irish Celts for some cross analysis.

The Gallic War: Seven Commentaries on The Gallic War with an Eighth Commentary by Aulus Hirtius (Oxford World Classics) by Julius Caesar: This book is one of the most extensive Classical primary sources regarding the Gaulish Celts.

Roman Cult Images: The Lives and Worship of Idols from the Iron Age to Late Antiquity by Philip Kiernan: This book takes an in-depth look at the interior of temple structures during the Iron Age and Roman Period of Western Europe (areas occupied by the Gaulish Celts). It talks about central concepts and themes regarding Gaulish ritual space and theological concepts that are attached to them and their iconography. This book is a MUST READ for anyone looking at a solid reconstructed understanding of Gaulish ritual space and reading this book along with Indo-European Sacred Spaces by Rodger Woodward will give you a solid understanding of IE sacred space themes.

Lady with a Mead Cup: Ritual, Prophecy and Lordship in the European Warband from La Tène to the Viking Age by Michael J Enright: This book is a break down of nomadic warband groups in Bronze and Iron Age Europe, specifically their development from the Mediterranean regions into central Europe with the Hallstatt and La Tène Celts and it’s migration into German culture, which eventually evolved into Norse Viking culture. Most of this book focuses on the warbands of the European Celts and talks about concepts of sovereignty, the importance of sacred drinking, and the role of Rosmertâ and Lugus in relation to Gaulish Celtic Culture and these warbands.

Celtic Curses by Bernard Mees: This book is more about early Gaulish poetic devices than it is about magical systems, although the fundamentals of Gaulish magical systems in relation to their religious practices are covered in this book to a certain degree. I would recommend reading Indo-European Poetry and Myth by Martian L West (I-EP&M) before reading this book because I-EP&M covers much of the foundational elements of IE mythic literature and themes and if you read Celtic Curses after I-EP&M, Celtic Curses feels like a continuation of I-EP&M. Both books are HIGHLY recommended…….

General Celtic Studies:

The Gods of the Celts by Prof. Miranda Aldhouse-Green PhD

The Ancient Celts by Berry Cunliff

The World of the Druids by Prof. Miranda Aldhouse-Green PhD

Ogam: Weaving Word Wisdom by Erynn Rowan Laurie

Celtic Myths and Legends by T.W. Rolleston (note: The strength of this text is the translations, which starts in Chapter 3. Chapter 1 has some solid information from historical sources, but you can skip chapter 2 since it focuses on how the academic community saw the Celtic culture in the early 1900s)

Various other Indo-European studies:

Indo-European Poetry and Myth by Martian L West: HIGHLY RECOMMENDED!!!

In Search of the Indo-Europeans by Prof. JP Mallory PhD

Myths and Symbols in Pagan Europe by H.R. Davidson

Hellenic Polytheism : Household Worship by  Christos Pandion Panopoulos and Vasilios Cheiron Tsantilas

The Norse Myths by Kevin Crossley-Holland

Indo-European Sacred Space: Vedic and Roman Cult by Rodger Woodard

Divination and Magical Studies:

Ancient Greek Divination by Prof. Sarah Iles Johnston PhD

Magic in the Ancient World  by Prof. Fritz Graf PhD

Magic and Superstition in Europe: A Concise History from Antiquity to the Present by Michael D Bailey

The Magic Arts in Celtic Britain by Lewis Spence

Trance-Portation: Learning to Navigate the Inner World by Diana L. Paxson

Local Research and Upper Midwest Folk Traditions:

Wisconsin Folklore by Prof. James P. Leary

The Living Great Lakes: Searching for the Heart of the Inland Seas by Jerry Dennis

Trolldom: Spells and Methods of the Norse Folk Magic Tradition by Johannes Björn Gårdbäck and Catherine Yronwode

Other Resources and Studies:

The Hearth Keepers Way: Publication by Ár nDraíocht Féin: a Druid Fellowship found here: https://www.adf.org/system/files/public/training/the_hearth_keepers_way.pdf

The Solitary Druid: Walking the Path of Wisdom and Spirit by Rev. Robert Lee (Skip) Ellison

Neopagan Rites: A Guide to Creating Public Rituals that Work by Issac Bonewits

The Essential Guide to Possession, Depossession, and Divine Relationships by Diana L. Paxson

Our Own Druidry: Publication by Ár nDraíocht Féin: a Druid Fellowship

Blood and Mistletoe: The History of the Druids in Britain by Prof. Ronald Hutton PhD

The Triumph of the Moon: A History of Modern Pagan Witchcraft by Prof. Ronald Hutton PhD

A World Full of Gods: An Inquiry into Polytheism by John Michael Greer