If you don't already know the names of all the various empires and subcultures of bronze age Europe, and if you happen not to carry in your mind detailed topographical and political maps of modern and ancient Europe, this book might not be the place to start. I have listened to most of this book at least twice and much of it five or more times, and I still got little more than bits and scraps of interesting stories from the text.
If, on the other hand, you enjoy pausing frequently from a book to read Wikipedia entries, this book is crammed with references worth learning about. The printed text contains beautiful visual aids, which are interesting, but which don't illucidate the writing sufficiently to make it accessible to somebody with less than an obsessive academic interest in the topic.
As much an epistemology as a history of the Celts, the author seems to compile and regurgitate various historical accounts of the Celts in a disjointed way and without much additional explanation or aid for modern readers. Not until the second half of the book does the author even start to give modern place names as points of reference.
This is frustrating, to say the least. I would expect a text on physics to entail prerequisite study, but this is a book about people and culture. It should be accessible to people, not just a hickorynut-shell-full of historians.
I am simultaneously reading Histories of Herodotus and getting much more out of it. Herodotus is a much better storyteller, and he writes his histories as chronological stories. Barry Cunliffe, by contrast, jumps here and there roughly following topics such as religion or war while traversing broad swaths of time and space, unloading the views of a handful of ancient historians with the occasional editorialization without explanations for the author's views.
Ancient geography is a prerequisite to the smooth reading of this book. Take the following for instance. "The geographer Hecataeus ... was aware that the Greek colony of Massalia ... lay in the land of the Ligurians near the territory of the Celts and that the settlement of Narbo (Narbonne) was Celtic."
Great. Where are ... or rather where were those places? Maybe my impoverished American education should shoulder partial blame, but I believe it's possible to communicate better a topic with which the audience is unfamiliar.
I lived for a year in Northern Italy and for several years in Switzerland, but I still had to google the Po Valley, which was referenced without so much as a pause to help the reader locate it mentally. Much of the geographical description merits a lengthy pause to consider the author's meaning.
Similarly, the author mentions one group of Celts preferring "impressed, or cardial pottery" but doesn't tell the reader what that means or looks like.
The search for the author's meaning is delayed further by the use of outdated termonology or terms from a less current language base, such as the Cynetoi for the Cynetes.
I had to chuckle also at the beginning of the book because the author warns against reading one's own cultural frame of reference backwards onto the Celts, especially for personal glorification. This warning bookends his name dropping of the view of Celtic research espoused by J. R. R. Tolkien, a fellow Oxford professor.
There is a lot of information in this book, albeit from the frame of reference of the author's particular worldview. However, I found the key to unlocking this information lay not in the book itself, but in Wikipedia. I listened to about 60% of the book, then started over, repeating the audiobook recording several times a minute and pausing for frequent Wikipedia searches. There are easier and more enjoyable ways to learn.