Frank Delany's The Celts is a "companion volume" to a BBC2 series produced in 1986. I don't believe that I ever saw the series, but the premise of the book (I was reading up on a lot of pre-Christian European cultures back then) was lure enough to order it, although it obviously wasn't enough to get it read until I needed a "change of pace" last month!
Because this is a "companion book" to a TV series, it lacks the coherency of a project specifically intended as a book, rather (I am assuming) following the pattern of the various programs in the series. The main sections are "Beginnings", "Nations", "Beliefs", "Expressions", "Credentials", and "Bequests", with brief interludes between each featuring some particular bit of Celtic myth and/or storytelling. The inclusion of these, admittedly, does "buffer" what might be a more disjointed information flow, but it also swings the "feel" of the narrative back and forth between modalities.
I suppose that one of the benefits of this being a book extracted from a TV series is that it feeds off the visual aspect, and includes many very fine illustrations, ranging from maps, aerial shots of ruins, classic art, sketches of archaeological sites, to a vast lot of museum photography giving very clear depictions of such treasures as the Basse-Yutz Flagons and the famed Tara Brooch.
As far as the book itself, it's really "quite a downer", being more a record of the fall (or progressive dilution) of Celtic culture than a celebration of it. This fall is, ultimately, blamed on the socio-political nature of the people, whose small tribal units spent more effort in inter-group warfare than in nation-making, so that when they were threatened by an outside force, be that the Anglo-Saxons, the Vikings, or Rome, it was almost impossible for them to form a united defensive front. In this theme, Delany traces the isolation (ala Ireland) or assimilation of what was arguably at one point "the Celtic world" (most of Europe) from the earliest invasions to the Irish diaspora to America, etc. He looks at various "Celtic revival" movements over the past 200 years and pretty much dismisses them all (the modern-day Druids, etc.) as rooted in fantasy, and disparages the few "official" attempts made in some areas. Frankly, from a perspective 20 years down the road, I think he might have been a bit off on this last point, at least from what I've seen from Chicago's "Celtic Fest" and the materials there from the Welsh (Cymru) tourist office ... from a few TV and radio programs that were attempting to preserve the (very difficult) Welsh language when this book was being written, there appears to have blossomed a rather robust "Celtic identity", a least in Wales!
While The Celts does appear to be out of print at this point (and, oddly enough, the BBC doesn't seem to have the series available either), the Amazon new/used vendors have "like new" copies of this heavily-illustrated hardback for as little as five bucks, and "good" copies for just $2.28 (plus shipping, of course), should a trip down the years to some possible ancestral culture prove an enticement to you.