Mari people have practiced some form of traditional pre-Christian religion to the present day, and the Republic of Mari El (along the Volga) recognizes it as an official religion. This makes it the only territorial entity in Europe that treats a type of paganism as an officially recognized religion.

They get occasional journalistic articles written about them, but there's not really any good academic or popular histories written about them. You might want to check r/anthropology to see if anyone there has any good resources.

Do you have a specific area of Europe in mind? Or a specific surviving religious tradition? All of Europe from the first conversion to Christianity to the modern day is literally an entire continent and nearly 2000 years to account for.