It’s a book about company. Macfarlane’s argument is that an old path is a record of everyone who wore it, that walking one puts you in their company whether you want it or not, and that this does something to the inside of your head — which is where he thinks a lot of thinking and most poetry actually comes from. So he goes looking for the oldest ways still walkable. The chalk of the Icknield Way. The peat roads of Lewis. An open boat on the sea roads to the Shiants. Six miles of tidal sand off Essex where the horizon disappears and the tide comes back faster than a man walks. His test case is Edward Thomas, who walked England compulsively, was talked into poetry by Robert Frost at forty, enlisted anyway, and died at Arras. The last chapters walk to where he fell.