Great addition to any history shelf covering what is and what is not known about the celts as a society. The general interpretation the majority of people have due to Roman literature was they were uncivilized barbarians, which proves to be quite contrary to the common historical understanding of the Celts. It admits there is much that is unknown and much more that is simply inferred about them but does a good job painting an accurate picture of what life was like as well as how their culture emanated throughout the region.
This reads very much like a high school history textbook and goes in about as much depth as you would expect a textbook to go into as well. It covers all major events with important details without regaling you with too many useless details. The majority of the first three-quarters of the book is dedicated to Celtic culture - housing, metalworking, societal structure, clothing, etc. The last few chapters describe the histories of the particular regions mostly in the context of their conquests and battles, leading to their eventual dissemination under Roman rule.
The last few pages summaries perfectly as to why this information is important - because Celtic society was incredibly varied and rich and that their influence endures in a countless way through modern society.