Synopsis from Goodreads.com: The Druids have been known and discussed for at least 2400 years, first by Greek writers and later by the Romans, who came in contact with them in Gaul and Britain. According to these sources, they were a learned caste who officiated in religious ceremonies, taught the ancient wisdoms, and were revered as philosophers. But few figures flit so elusively through history, and the Druids remain enigmatic and puzzling to this day. In this Very Short Introduction, one of the leading authorities on British archaeology, Barry Cunliffe, takes the reader on a fast-paced look at the ever-fascinating story of the Druids, as seen in the context of the times and places in which they practiced. Sifting through the evidence, Cunliffe offers an expert’s best guess as to what can be said and what can’t be said about the Druids, discussing the origins of the Druids and the evidence for their beliefs and practices, why the nature of the druid caste changed quite dramatically over time, and how successive generations have seen them in very different ways.
Review: This is one of these books that give you the minimum of information on a subject that may be interesting to you but you are not sure if you want to go into depth about it. It would make a great book for people who want to put together a starter kit for someone who is looking into subjects to study but are not sure yet if this is the thing they want to pursue.
The book is interesting in that the author tried to differentiate between the literary and archeological evidence and present each one separately. He also showed the context in which these literary sources were written. He also showed how the Druid caste changed from the years between 400 BCE and 400 CE and how the society of which they were a part also changed.
There might be something in this book that might come as a surprise to people who have been studying the Celts. The author has been putting forward the theory that the Celts had actually originated in Atlantic Europe as opposed to the theory that they spread from Central Europe. This has been put forward as a theory by Cunliffe in 2001 and then seems to have gained some momentum with the finds by John T. Koch in his book Tartessian: Celtic in the South-west at the Dawn of History (Celtic Studies Publications). In this book Professor Cunliffe is making the assumption that the Celts originated in Atlanitc Europe and presents his findings with that in mind.
I really enjoyed this book as I have enjoyed everything that Professor Barry Cunliffe wrote. The further reading part of the book is amazing in that it breaks down the most important books you can read on the Celts, the Druids, and the Celtic Religion. Even though this is the minimum you can know about the Druids it gives a very good basis for which you can launch your own research if you like or say that you know about the Druids and leave at that if that is your wish.
Celtic Scholar's Reviews and Opinions 
Nice post. It’s easy to romanticize about the Druids but Caesar wrote about them and was convinced they partook in human sacrifice. The only way they could please the gods was to kill people, criminals if possible but if not, someone had to get it.
“The whole Gallic nation is virtually a prey to superstition, and this makes the serious invalids or those engaged in battle or dangerous exploits sacrifice men instead of animals. They even vow to immolate themselves, using the Druids as their ministers for this purpose. They feel that the spirit of the gods cannot be appeased unless a man’s life is given for a life.
Public sacrifices of the same sort are common. Another practice is to make images of enormous size, with the limbs woven from osiers [willows]. Living human beings are fitted into these, and, when they are set on fire, the men are engulfed in the flames and perish. The general feeling is that the immortal gods are better pleased with the sacrifice of those caught in theft, robbery or some other crime. But if a supply of such criminals is lacking, then they resort to the sacrifice of completely innocent victims. . . ”
How much of this is true and how much was propaganda is difficult to say, but the chances are there was some truth to the accusations.
http://www.primarysourcebook.com/ancient/human-sacrifice-and-the-druids-by-julius-caesar