This is a Dover-style reprint-illo book -- not as well-documented or professionally-produced as the Dover books, but the imagery reproduced here is so compelling that the book is well-worth a look.
For instance, here's the Gundestrup Cauldron (circa first century BC, photo at Amazon). This huge silver vessel, found dismantled in a Danish peat bog, is filled with enigmatic images of Celtic deities and rituals. See Wikipedia for the details, which you won't find in this book. But the photographs here are excellent.
Which is the pattern for the book: tantalizing photos of really cool artifacts, like the Battersea Shield (see photo at Amazon), dredged from the Thames, where it was likely placed as an offering, some 2000 years ago. With just enough information to tantalize (or frustrate) the reader. Fortunately, you can easily research the ones that particularly interest you online.
So: the quality of the photographic reproductions is variable -- but many of these artifacts are simply wonderful, and many were new to me. The paper quality is good, and the book is inexpensive. Cautiously recommended to Celtic art fans.