In Benjamin Percy's new thriller, a post-apocalyptic reimagining of the Lewis and Clark saga, a super flu and nuclear fallout have made a husk of the world we know. A few humans carry on, living in outposts such as the Sanctuary-the remains of St. Louis-a shielded community that owes its survival to its militant defense and fear-mongering leaders. Then a rider comes from the wasteland beyond its walls. She reports on the outside world: west...