Fourth The Good Earth.
Nominate The Grapes of Wrath by John Steinbeck.
From Wikipedia (to those few who may not be familiar with this novel)
The Grapes of Wrath is an American realist novel written by John Steinbeck and published in 1939. For it he won the annual National Book Award and Pulitzer Prize for novels and it was cited prominently when he won the Nobel Prize in 1962.
Set during the Great Depression, the novel focuses on the Joads, a poor family of tenant farmers driven from their Oklahoma home by drought, economic hardship, and changes in financial and agricultural industries. Due to their nearly hopeless situation, and in part because they were trapped in the Dust Bowl, the Joads set out for California. Along with thousands of other "Okies", they sought jobs, land, dignity, and a future.
It's available as an ebook.