When he finished the 750-page manuscript he had been writing for five months, John Steinbeck had grave doubts about whether the resulting book could be a commercial success.

He wrote to his agent warning her not to let his editor "oversell you on this" manuscript. He wrote to his new publisher, Viking, suggesting that they cut back on plans for a large first printing.Undissuaded, Viking published 50,000 copies of "The Grapes of Wrath" on April 14, 1939, 50 years ago. (Viking has just published a 50th anniversary of the book, selling for $25.)

"The Grapes of Wrath" became a No. 1 best seller and went on to sell 14 million copies worldwide. It won the Pulitzer Prize for fiction in 1940 and was cited as Steinbeck's major work when, in 1962, he became the sixth American to win the Nobel Prize in literature.

From its publication, Steinbeck's panoramic novel about displaced Oklahoma dust bowl farmers and their migration to California was embroiled in controversy. Angered by Steinbeck's depiction of the exploitation of migrant field workers, the Associated Farmers of California campaigned against the book. So did Oklahoma newspapers and legislators, who took offense at Steinbeck's portrait of the "Okie" tenant farmers. Shocked at the coarse language, librarians across the country banned the book as obscene.

The controversy has vanished, but the book endures - as a popular classic, if not really a literary one. Steinbeck has never had an exalted reputation among the critics, who have generally considered his books too programmatic, sentimental and heavy-handed, devoid of irony, alienation and narrative disengagement. He lacks the concise style of Hemingway, the complexity of Faulkner, the elegance of Fitzgerald, the psychological complexity of the European modernists.

Yet to dismiss Steinbeck because of his shortcomings is to fail to acknowledge his distinct, if unfashionable, strengths, which are nowhere more evident than in "The Grapes of Wrath." In this age when writers so often hold their characters in contempt, Steinbeck's attitude seems quaintly ingenuous.

"I grew again to love and admire the people who are so much stronger and purer and braver than I am," he wrote in the journal he kept while composing "The Grapes of Wrath." (Viking has just published for the first time "Working Days: The Journals of `The Grapes of Wrath,' 1938-1941," edited by Robert DeMott.)

Steinbeck's social commitment ("My whole work drive has been aimed at making people understand each other") and his faith in the fundamental dignity of American working people can sound simplistically pious or naive.

But like his populist contemporaries, the poet Carl Sandburg and the composer Aaron Copland, Steinbeck expresses simple themes within an imaginative yet sturdy and quite accessible artistic structure. "The Grapes of Wrath" is built on a simple but effective scheme of 30 alternating chapters: The even-numbered chapters follow the Joad family's migration from Oklahoma to California; the odd-numbered chapters present general sketches of scenes of migrant life.

The book has been called realistic, and it is so, but only in that Steinbeck based the novel on a contemporary social event and he took some pains traveling and working among the migrants in California before writing the book to ensure the accuracy of the details about migrant worker life. (Steinbeck never made the westward migration from Oklahoma, which makes up the first half of "The Grapes of Wrath.")

But accurate details alone do not make for realism in overall technique. More accurate would be to say that Steinbeck uses realistic material in a non-realistic way.

"My people must be more than people," Steinbeck wrote in his journal. "They must be an overessence of people."

In most successful novels we feel we get to know the characters better as the novel progresses, but in "The Grapes of Wrath" we know the characters upon introduction, and as the book progresses they become not more lifelike but larger than life. By the end we see the Joads not as just one suffering family but as a fictive embodiment of an entire caste of displaced people.

"The Grapes of Wrath" made many people aware for the first time of conditions in the California migrant camps - the issue had been largely ignored by the press - but the book didn't really lead to reforms. Conditions and wages improved when the war made labor scarce.

Today illegal immigrants rather than legal migrants bring a different set of problems to the Southwest, and perhaps in their story, which has been treated movingly in films such as "El Norte," lies another great book waiting to be written.

It's unlikely that book will be a novel, for today's great books on vast social themes tend to be what might be called non-fiction epics: J. Anthony Lukas's "Common Ground," which alternated foreground and background chapters in its look at how school desegregation affected three Boston families, and Neil Sheehan's "A Bright Shining Lie: John Paul Vann and America in Vietnam" are two that come to mind.

But to say they don't write novels like "The Grapes of Wrath" anymore simplifies the point. They didn't write many like it 50 years ago either, and that's why it remains an important, moving and original work today.