The Stevenson in question is the Scottish author Robert Louis Stephenson, best known in anglophone countries for Treasure Island and The Strange Case of Doctor Jekyll and Mr Hyde. In Monastier-sur-Grazeilles, in the French Massif central, however, he is principally known as the author of Travels with a Donkey in the Cévennes.
Stevenson spent some time in the small town of Monastier-sur-Grazeilles before setting out on his walk across the southern half of the French Massif central. Despite the title of his book, Stevenson set off from the Velay area, crossed the Gévaudan, and climbed Mont Lozère before even entering the Cévennes.