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What I'm finding indicates that the decision was not overturned, for that to have happened, it would have had to go before the Supreme Court and did not. So the result is that while you may own a letter from, say, John Irving, Irving owns the language in it; if you wish to discuss it without his permission you have to paraphrase. Argh, some fair use that is.
The privacy-obsessed should note that while Salinger was concerned about these old letters which showed how he was 40 or 50 years ago, an unintended side effect of Salinger's suit (intended to protect his privacy) was that a lot of detail of Salinger's current and very much private life became irrevocably public (his testimony, and everyone else's, was transribed and made available in print to anyone at all who wanted a look). Some of that info included that he had written at least two novels and several stories. Anyway, but so to see those letters, eh? Some of them were to Hemingway....
Rather interesting discussion [here], for anyone interested. --KQ