Nobody knows exactly how long the Appalachian Trail is. It’s about 2,200 miles as the shoe walks, from Springer Mountain in Georgia to Mount Katahdin in Maine, but the length changes constantly as the trail is rerouted in response to flood, fire and traffic. Official guides have variously given the length as 2,144 miles, 2,159 miles and “more than 2,150 miles.” In “The Appalachian Trail: A Biography” Philip D’Anieri, a lecturer in architecture and regional planning at the University of Michigan, provides deft, engaging profiles of a dozen people who were instrumental in the trail’s history. These range from Arnold Guyot, who literally put the Appalachian Mountains on the map, to Bill Bryson, whose bestselling “A Walk in the Woods” resulted in a 45% increase in “thru-hikers”—those who travel the entire length of the trail in a single trip. Not bad for a travel writer who walked only 10% of the trail.