Fifty years ago, John Steinbeck's now classic novel, The Grapes of Wrath, captured the epic story of an Oklahoma farm family driven west to California by dust storms, drought, and economic hardship. It was a story that generations of Americans have also come to know through Dorothea Lange's unforgettable photos of migrant families struggling to make a living in Depression-torn California. Now in James N. Gregory's pathbreaking American Exodus, there...