“The Nobel Prize for Literature, one of the six international awards administered by the Nobel Foundation, honors outstanding achievement in letters.”
It is NOT given for specific works.
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From page 915-6 of Benson's Steinbeck biography we have this from the 26 October 1962 New York Times ...
The award of the Nobel Prize for Literature to John Steinbeck will focus attention once again on a writer who, though still in full career, produced his major work more than two decades ago. The award will bring back the vivid memory of the earlier books, the relaxed gaiety of "Tortilla flat"; the stark force of "In Dubious Battle"; the anger and compassion of "The Grapes of Wrath," a book that occupies a secure place as a document of protest.
John Steinbeck will go down in history of one of Earth's greatest writers.