I grew up in Oklahoma until I was 15 so had certainly heard about the Dust Bowl and the migration of "Okies" to California. Egan attempted to personalize these events by interviewing survivors, drawing from diaries, newspaper accounts, and government documentation. The scale of the disaster, caused largely by human behavior really was staggering--millions of tons of dust was blown away. In the process, people died of "dust pneumonia," lost their crops, livestock, land, and way of life. Egan does a good job of painting the plains as the home of bison and Indians, with grasslands that held down the dirt for thousands of years of human habitation. But then came the wholesale slaughter of the bison to deprive the natives of their food source, followed by ranchers who fenced the land, and finally--fatally--by farmers who tore up the grass to plant wheat. So that when the drought came, as they historically always do, and the crops died, there was nothing holding the earth in place. Even when people could get small crops to grow, the hordes of grasshoppers ate everything down to the ground.
The facts and figures are consistently astounding. And a few characters emerge as memorable. But largely I found this book tried to do too much, talk to too many people, and portray too many stories to make it a compelling read. It felt choppy and slow, with frequent shifts in the locale and time frame of the narrative. Egan may well have had his journalistic reasons for such choices, but I have not enjoyed reading his books, unlike those of Erik Larson, who does basically the same genre. Nevertheless, Egan won a National Book Award (2006) for this tome, so it's just my opinion. More detail and positive reviews at Kirkus and the New York Times.