By age 25, Heather Anderson had hiked what is known as the "Triple Crown" of backpacking: the Appalachian Trail (AT), Pacific Crest Trail (PCT), and Continental Divide Trail (CDT)a combined distance of 7,900 miles with a vertical gain of more than one million feet. A few years later, she left her job, her marriage, and a dissatisfied life and walked back into those mountains. In her new memoir, Thirst: 2600 Miles to Home, Heather, whose trail name...