“It is impossible to return the full and authentic remains of Monsieur Yeats,” wrote Bernard Cailloux, the French diplomat who was dispatched to Roquebrune-Cap-Martin, on the Côte d’Azur, 75 years ago to exhume and repatriate whatever was left of the Irish Nobel laureate, nine years after the poet’s death. Somehow, in the run-up to the second World War, William Butler Yeats had been consigned to a pauper’s grave, then an ossuary, where bones were sorted by type: skulls with skulls; tibiae with tibiae, femurs with femurs. Were it so required, Cailloux continued, the local sworn pathologist “would be capable of...