Once again, I'm plowing into a series of books here ... again from my book buying days in the early 90's, and again from Time-Life. These are four books from their "Lost Civilizations" series, which is really quite well done. This volume, The Celts: Europe's People of Iron takes a look at the iron-age Celtic cultures across Europe in the first millennium B.C.E.
The "Lost Civilization" series is interesting in that, while hardly being academic, is very well researched, and extensively illustrated ... rarely is a key artifact mentioned without being pictured, which helps one "experience" the information. While brief (each is about 160 pages), each volume tackles its subject in some breadth, and the books are somewhat reminiscent of a museum guide book produced for a themed exhibit.
While I've read a goodly amount of Celtic historical stuff (ala Pagan Celtic Ireland a number of weeks back) which has gone into more detail, The Celts: Europe's People of Iron provides a wider perspective, discussing cultural remains from Spain to Turkey to Ireland. Because the Celts didn't leave written records (the Druids forbade the non-verbal transmission of their knowledge, and it was only after Rome's conquests that some of this material was set to paper), didn't (generally) build major enduring stone edifices, and typically practiced cremation (leaving few "rich tombs"), their culture, spanning all of Northern Europe for a thousand years, is largely only known from outside sources and few "fortunate" survivals of ritual sites.
This book looks at these various materials, and discusses the major figures who researched the Celts over the past several centuries. Again, this is hardly a college course on the Celtic World, but I felt it was a very decent over-view of the subject, which put the culture of the Celts into historical context and provided insights into their economy, art, and religion that would be hard to get without a lot of museum visiting! While being out of print, copies in various conditions can be had from the Amazon new/used vendors for as little as $1.30 ... if you're interested in learning about the Celtic culture, this would be a good one to pick up!