This is not a rigorous archeological investigation of a culture or people. The author relies on literary sources, particularly Roman ones, to relate how external cultures perceived various Celtic groups. There appear to be good maps, but the author doesn't tie any investigation by historical sequence to the maps presented.
As opposed to more recent in depth analysis of Norse and Viking culture, where elements that constituted a lifestyle are presented in a context where those elements expand to the translation of a community and then governmental group, complete with trade rules, routes, merchandise, equipment, and time span within the encompassing external cultures, this book goes to glimpses from the external cultures to give snapshot impressions of the people (Celts) being investigated.
The information is presented as chapters which deal with disparate and unconnected information regarding regions and time spans. There is no effort to tie all the information together. Because of the method of subject treatment, there is no smooth chronology.
This entire works appears to be scrapings from other research or from literary sagas/myths, where this author is just passing glimpses along.
Since I'm at a loss to fully describe my disappointment with this work, I'll have to use an analogy. Suppose that you are studying to be a vet and are particularly interested in the giraffe. You might expect a volume of work concerning the animal to give you various statistics such as bone structure, body mass density, blood pressure, feeding habits, mating and reproductive characteristics, range over time, subspecies variations, etc. Picture instead a work that gives you a narrative view of an entire zoological safari, in which the giraffe is only seen through interactions with the other animals, and in which the animal namings and residence areas obscure the storyline of what is seen and told of the giraffe. Nearly all direct references to the subject come from literary stories and not from actual first hand on site physical presence.