This is the one that all other books are judged by. Piggott doesn't wander off into romanticism but it also isn't just dry anthropology.
In "The Druids" Piggott first defines the limits of what can be known about any pre-literate people such as the Druids, and how it can be known. This is not, as so many other books on the topic turn out to be, a romantic description of an ancient people, but rather a history first of the archeological, then the contemporary historical, and finaly the historiographic records of the Druids, who they may have been, and what they may have been about.
Throughout, Piggot continuiously contrasts the three levels of historical knowledge, described as "Druids as known," "Druids as inferred," and "Druids as wished-for." The first third of the book is one of the better examples of conservative (not in a political, but an academic sense) archeological interpretation as Piggot explains the few hard facts that can be discerned from material remains, and the few inferrences which can be made from those facts. To romantics and lovers of "Druids as wished-for" this part will probably seem quite dry and lifeless, but for those interested in real archeology and it's interpretation, this may be the most interesting part of the book.
Piggott then takes us on to contemporary accounts of the Druids from their literate neighbors in the ancient world, while still mainting his contrast between what is known and what is inferred. This is the part of the book that will interest those who want to get the best possible picture of who and what the Druids really were, as these contemporary and near contemporary accounts are the closest things we have to real insight to the culture of the ancient Celts. Of course, this can be gleaned from the primary sources as well, and whether or not you read Piggott's book, Ceasars "Gallic Wars" and the Ulster Cycle of Irish legends are both well worth reading.
Finally, Piggott takes us on a history of the history of Druids, from the Renaissance through the modern era. This is almost as fascinating to me as the prior two-thirds! of the book, as this section shows how the scant historic and archeological records were interpreted in all manner of romantic, nationalistic and downright bizarre ways.
All in all, "The Druids" is a fine example of sussing out the real history of a subject that is often treated with more romance and fantasy than scholarly rigor. Unfortunately, there is little that we can truly know about the Druids because they left no written records themselves, and their neighbords had all manor of political and cultural biases when they wrote about them. But even if the Druids still remain a cypher, isn't it far more fun, and more fascinating to try to solve the cypher with what facts are available, rather than the romantic fantasies that have piled up over the years?