For much of a year, writer Sebastian Junger and three friends—a conflict photographer and two Afghan War vets—walked the railroad lines of the East Coast. It was an experiment in personal autonomy, but also in interdependence. Dodging railroad cops, sleeping under bridges, cooking over fires, and drinking from creeks and rivers, the four men forged a unique reliance on one another. In his new book “Freedom,” Junger weaves his account of this journey together with primatology and boxing strategy, the history of labor strikes and Apache raiders, the role of women in resistance movements, and the brutal reality of life on the Pennsylvania frontier. The book is a powerful examination of the primary desire that defines us.