Most of this book is dry as a bone. I'm sure the information in here is accurate, but it wasn't fun reading it, and in fact I abandoned it about 200 pages in.
Some highlights of what I did like:
1. The number one best part of this book for me was a discussion of why the Roman Empire expanded Northward into Celt country (as opposed to Southwards into sub-Saharan Africa, or Northeast to the sparsely populated area now known as Ukraine). The main factor is that the Celts traded goods rich in tin- an ingredient of the wonder-metal of antiquity, Bronze. Italy and the Middle East (regions of Roman power) are relatively tin-poor, but Northern Europe, and in particular Great Britain are rich in tin.
All of this has nothing to do with the Celts or Celtic culture; it's more of a geography/geology tidbit, but it's still interesting.
2. The author describes the wide variation in habits and aesthetics amongst the Celtic people, who spanned from Ireland to Bulgaria. I'm still not sure what exactly makes something Celtic, since the respective cultures of Ireland and Bulgaria look very different to me, but maybe the topic of what constitutes Celt-ness is covered in the chapters I decided to skip. My loss, I'm sure.
This is probably good for an academic, or somebody with a lot of previously-accumulated knowledge about the Celts, but not so great as an introductory text.