Reddit reviews A History of Pagan Europe

I'll have to check that one out myself.

A lot broader (and a little pricier) is "A History of Pagan Europe" by Nigel Pennick and Prudence Jones, but you can preview a decent amount on Amazon. If you go to Table of Contents and click on Chapter 5, that has some basics about the Celts (including the Helvetii) and touches on some similarities and differences between Celtic and Germanic traditions: https://www.amazon.com/History-Pagan-Europe-Prudence-Jones/dp/0415158044

"Myths and Symbols in Pagan Europe" might be worth it, too; if you buy a used paperback it's not too expensive. If I remember correctly, it's a bit more dense, but both of these books are fairly academic: https://www.amazon.com/Myths-Symbols-Pagan-Europe-Scandinavian/dp/0815624417

I've only read bits and pieces of both, so I'm not comfortable recommending them, exactly, but they might have some of the info you're looking for.