My birth father was part of the great migration from Oklahoma to the western states during the Depression era years, the 1930s. When I went looking for books to read, I came across Southern Diaspora: How the Great Migrations of Black and White Southerners Transformed America, a companion website to the book authored by James M. Gregory, a University of Washington faculty member.
The Southern Diaspora may have been the most momentous American population movement of the twentieth century. Between 1900 and 1980 more than 20 million southerners left their home region looking for jobs in the cities, suburbs, and farms of the North and West. Most visible were the African American southerners whose migration transformed urban America and set the stage for important changes in racial understandings and the rights of people of color. White southern migrants outnumbered black migrants and in some settings were almost as controversial. Called "hillbillies" in the North and "Okies" out West, the whites faced challengesdifferent than most Americans who move across state lines.
The website contains oodles of starting points for further research: photos, tables, other links, and my favorite, the bibliography. With Gregory's help, you can easily go beyond Grapes of Wrath in understanding your Depression era relations, and find a plausible reason for your great granddad ending up in Detroit after leaving the old family home in Yazoo City (Gateway to the Delta).