All the books and articles that I have previously read about the Celts have stressed the archaeological evidence. They have especially examined the art, culture, and daily lives of the ancient Celts, and their origins as a people. Freeman, instead, looks at how the Romans and, especially, the Greeks saw them. So, of course, there are a lot of quotes about the Celts as bloodthirsty drunken louts.
But once I got beyond that, there was a lot I hadn't learned from the books and articles I had read earlier. For example, there is more about the Celts of Iberia, Asia Minor, and Cisalpine Gaul (Northern Italy). The Galatians of Asia Minor seem to have been especially important as mercenaries to rulers of the Eastern Mediterranean. There is also more about Caesar's conquest of Gaul. I had previously read about this mostly from Caesar's point of view (in Latin). Freeman counters with a description of how well Vercingetorix and his people resisted Caesar, although they ultimately lost.
Posidonius, a Greek aristocrat, visited only the upper crust of Celtic society, and so we don't see much of the lives of ordinary Celts. In this he is more like a historian than like an archaeologist. He is the philosopher of the title and the unifying symbol of the whole book.
In the latter half of the book, Freeman drops the emphasis on the ferocity and barbarity of the Celts and says more about their mythology and religion, the position of women, and the social lives of the upper crust. This is very speculative and that's probably why other serious authors haven't had so much to say about it. But if you read it as a commentary on what the Celts might have been like, and how their neighbors reacted to them, it will give you a lot to think about.
So, for what Freeman says that the other writers don't say, and especially if you've read some of the other books, I recommend this one.
[This review reflects my awareness of my Celtic ancestry. If you're interested, click on my name, above.]
This is the most readable introduction to the Celts I've ever seen. It takes the real journey of a Greek philosopher to Celtic lands in the days before Julius Caesar and uses it as a connecting thread to tell the whole story of the ancient Celts. I liked the way it was organized around different Celtic themes, like druids, women, war, etc. And the way it ties the ancient Celts into Irish culture is great.