After Z Day — Eric S. Brown

The dead rose from their graves, hungry for the flesh of the living. Civilization crumbled in a matter of days. But in the wake of the apocalypse, there are those who struggle to survive. Colonel Porter and her unit, now a mix of military personnel and tag along civilians, stay on the move. Looting what they can from the corpse of the old world, they head westward into the American Heartland in search of somewhere safe where they can stop their endless flight. However, the apocalypse was just the beginning and the worst is yet to come.