"Two friends, zero preparation, one dream: a 750-mile odyssey, on foot, through the heart of America's most magnificent national park and the grandest wilderness on earth. A few years after quitting his job to pursue the ill-advised ambition of becoming a whitewater guide on the Colorado River, Kevin Fedarko was approached by his best friend, the National Geographic photographer Pete McBride, with a vision as bold as it was harebrained. Together,...