John Steinbeck performed a rare feat for a writer of fiction. He created a literary portrait that defined an era. His account of the “Okie Exodus” in The Grapes of Wrath became the principal story through which America defined the experience of the Great Depression. Even today, one of the enduring images for anyone with even a passing familiarity with the 1930s is that of Steinbeck’s fictional characters the Joads, an American farming family uprooted from its home by the twin disasters of dust storms and financial crisis to become refugees in a hostile world. Not since Dickens’s portrayal of the slums of Victorian England has a novelist produced such an enduring definition of his age.

According to Penguin Books, which produced a very handsome series of paperbacks to mark the centenary of his birth this February, Steinbeck’s novels still generate a combined sale of around two million b ...