- The Celtic Gauls: Gods, Rites and Sanctuaries; Jean-Louis Brunaux; 1988.

This a study of Gaulish religious features, in their public and ritual aspects, but also how they might have conceived their spiritual universe and their Gods; but also the relation religion had with society, particularly warfare and druidism (taken the author as not as much a religion than a mix of philosophy and theology).

The comparison with Mediterranean peoples and especially ancient Greeks provides with an interesting perspective of the author, who stresses a Mediterranean aspect of Gaulish civilization. The author wrote more books about religion and philosophy in Gaul, which are benefiting from linguistic and archaeologic data from the last decades, but they are in French.

(Eventually, being able to read French, as recent secondary sources on the topic of Gaulish religion in particular can be left untranslated more often than not, might be useful)

- Gods of the Celts; Miranda Green; 1986 (reed. 2016)

It's definitively a different, and arguably opposite, perspective on Celtic religion there : religion being untouched by Mediterranean influences, Celtic gods being only partly romanized after the conquest. Developments on animism and especially the consideration of Celtic religious and spiritual aspects outside Gaul (especially in Roman Britain and medieval Welsh and Irish sources) does interestingly covers a more broad "panCeltic" vision with Druids as priests.

Barry Cunliffe did a Druids book for the Very Short Introduction series, whose first chapters sound right up your alley in terms of interest. The writing is clear and--as the series suggests--definitely aimed at non-experts (hi!), but includes a lot of quotes from primary sources.

It's available on Audible, too, if that interests you.