Although this book is undoubtedly out of date, published in 1971, it’s a fascinating survey of what was known and believed about the Celts at the time. Some of the theories are less in vogue now (with more credit given to the spread of ideas than the spread of people for the changes in agriculture, art, etc), but descriptions of the archaeology, art and literature are solid and worth reading. I found Chadwick’s style very pleasant and easy to read: this is serious and somewhat academic in depth, but not boring.
Pretty much my only quibbles, when you lay aside the outdated theories, were the way the literature was described at length. I don’t need a description of Táin Bó Cúailnge — I’ve read it! And my other quibble would be the intense focus on Ireland. It does make sense within the frame Chadwick gives us, where Ireland was more conservative in culture and thus retained purer Celtic culture for longer, but I would still love to have read as detailed a discussion of the Welsh texts surviving, particularly stuff like the Triads.
If you read it knowing that, of course, other theories are in vogue now and some of it has been disproved, it’s a pretty sober and admiring look at Celtic culture. Maybe a touch too much judgement re: civilisation vs barbarism, with the Celts decidedly on the latter end, but there’s still admiration, and no prurient focus on the idea of ritual human sacrifice (which, judging from this, was not considered common then either).