A review by gotossmycausticsalad
Blood and Mistletoe: The History of the Druids in Britain by Ronald Hutton

Did not finish book. Stopped at 44%.
If you want to learn everything about how the druids lived, their beliefs and their behaviours, this is perhaps not the book for you. 
But neither is any book, as this one so neatly outlines. The first chapter is dedicated to all the things we know about the druids for sure, and even then it provides few definitive answers. 
To summarise and paraphrase: 
"Here is a historical artifact linked to the druids. Here are two completely opposing theories as to what it tells us about the druids. We have no way to prove either way. 
"Here is a historic text relating the the druids. Here are two violently opposing theories. Either is as likely to be true as the other, why not flip a coin.
"The opinions of the leading scholars on the druids are as likely to be valid as each other and based pretty much arbitrarily on their individual biases and belief systems."

The rest of the book is no less interesting, exploring the way different cultures related to the druids - what they thought of them and what that might show us about their own societies. Unfortunately I was unable to finish the book through no fault of its own - I got covid in the middle of reading, couldn't concentrate enough to read non-fiction and had to return it to the library before I could pick it back up again.