With brief details regarding historic events and archaeological finds relevant to each map, this beautiful book from 2001 provides an excellent overview of the conjectured beginnings, piecemeal expansion and decline of the “Celts” (however that historical label may be interpreted).
After an introductory essay about Celtic identity, there are two main sect on Continental Celts and Atlantic Celts.
The clear maps with useful explanations describe Continental Celtic developments in a broadly chronological order, including the original Celtic expansion with the movement of tribes, the defeat and absorption of Continental Celts by Roman military forces and the invasion of Germanic tribes, and final kingdom of Brittany until it is absorbed into France.
The second section on Atlantic Celts dwells for only a couple of maps on pre-Roman Celtic Britain, and mainly illustrates the gradual withdrawal of Britons from Anglo-Saxon England, followed by the anglicisation of Wales, Scotland and Ireland. For me these maps were most useful for clarifying how gradual this process was, how the Picts died out and Irish Gaelic became the language of the Scottish Highlands, and the English “plantations” in Ireland. Although all of this history is a lot more familiar to me, the use of maps makes these processes much easier to understand. In particular the assimilation of the Picts and the Highland clearances.
The final section relates to the modern (post seventeenth century) creation of a Celtic identity, arising from initial attempts to understand archaeological ruins and cultural ideas such as romanticism. This is fascinating as looking at cultural developments from self identification, rather than historical basis.

I bought this book after visiting the excellent exhibition on the Celts at the British Museum in 2015, and this has provided a useful refresher for the issues raised in that exhibition. ( )