BEWCASTLE, England — The road leading north from Carlisle through the low, windswept hills stretching to the Scottish border traces Hadrian’s Wall, once a remote outpost of the Roman Empire. Nearby is the tiny outcropping of Bewcastle, where a broken Celtic cross with one of the oldest sundials in England stands in a lonely churchyard.

This former Celtic settlement in northwest England tends to draw a New Age-y crowd, the author Graham Robb noted, as he traipsed around the churchyard on a recent blustery afternoon. Such places are at the heart of his new book, “The Discovery of Middle Earth: Mapping the Lost World of the Celts,” in which he peels back the layers of Europe to reveal the traces of Celtic civilization. (The book is called “The Ancient Paths” in Britain.)

A literary historian best known for his biographies of French authors and books about France, including “The Discovery of France” and “Parisians, ” Mr. Robb says that in this work he stumbled on an understanding of how astronomy guided the Celts, semi-nomadic tribes who dominated Europe in the Iron Age, from 800 B.C. to A.D. 600, in their migrations from the Iberian Peninsula to the Black Sea....