The Stand by Stephen King
(used, paperback)
When a man escapes from a biological testing facility, he sets in motion a deadly domino effect, spreading a mutated strain of the flu that will wipe out 99 percent of humanity within a few weeks. The survivors who remain are scared, bewildered, and in need of a leader. Two emerge–Mother Abagail, the benevolent 108-year-old woman who urges them to build a peaceful community in Boulder, Colorado; and Randall Flagg, the nefarious “Dark Man,” who delights in chaos and violence. As the dark man and the peaceful woman gather power, the survivors will have to choose between them–and ultimately decide the fate of all humanity.