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.
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.
Lady with a Mead Cup: Ritual, Prophecy and Lordship in the European Warband from La Tene to the Viking Age by Michael J. Enright: This book is an in depth look into Celtic and European war bands of the Hallstatt Celts and the later Germanic culture in relation with neighboring cultures such as the Greeks.
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:
In Search of the Indo-Europeans by Prof. JP Mallory PhD
Myths and Symbols in Pagan Europe by H.R. Davidson
Death, War, and Sacrifice: Studies in Ideology & Practice by Bruce Lincoln
Hellenic Polytheism : Household Worship by Christos Pandion Panopoulos and Vasilios Cheiron Tsantilas
The Tradition of Household Spirits: Ancestral Lore and Practices by Claude Lecouteux
The Norse Myths by Kevin Crossley-Holland
Divination and Magical Studies:
Ancient Greek Divination by Prof. Sarah Iles Johnston PhD
Magic in the Ancient World by Prof. Fritz Graf PhD
The Magic Arts in Celtic Britain by Lewis Spence
Trance-Portation: Learning to Navigate the Inner World by Diana L. Paxson
Trolldom: Spells and Methods of the Norse Folk Magic Tradition by Johannes Björn Gårdbäck and Catherine Yronwode
Local Research and Upper Midwest Folk Traditions:
The Living Great Lakes: Searching for the Heart of the Inland Seas by Jerry Dennis
Immortal River: The Upper Mississippi in Ancient and Modern Times by Calvin R. Fremling
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