“Compilations of correspondence are necessarily biographies of a kind—biographies of individual consciousness with less intrusive mediation and interpretation than one finds in a traditional biography,” Peter Y. Sussman, editor of “Decca: The Letters of Jessica Mitford” (Alfred A. Knopf, 744 pages, $35) writes. But what constitutes “less intrusive mediation”? Jessica Mitford supplied an admirable answer, which Mr. Sussman quotes: “The whole point of letters is to reveal the writer & her various opinions & let the chips fall where they may. Censoring them for fear of offending the subjects is in my view absolutely wrong.”

Why then does Mr. Sussman disregard Mitford’s uncompromising conviction? Mitford belonged to a family of outspoken individualists, including her older sister, the novelist Nancy Mitford, who satirized her own family’s peculiarities and their devotion to the fascist cause. Another sister, Diana Mosley, was unapologetic about her marriage to the British fascist Oswald Mosley, and Jessica herself publicly excoriated Diana and another sister, Unity, for their pro-Hitler activities. Jessica, for her part, became a communist and later an outspoken critic of American institutions, and is perhaps best known for her watershed book, “The American Way of Death,” a hilarious but savage attack on the funeral home industry.

What is more, unlike many of her fellow writers, Mitford applied her principles to herself and to her friends, sparing (with one exception) no person or organization when she believed an important principle was involved. For example, when biographer Joan Mellen asked for an interview with Mitford about her friend, the writer Kay Boyle, Mitford assented even when the capricious Boyle withdrew her support for Mellen’s biography and enjoined Mitford to do the same. Mitford refused, preferring to anger her friend, and to honor an agreement, noting, as well, that she had a right to speak with anyone she liked. Would that more writers obeyed that Samuel Johnson injunction: It is more important to reveal the truth than to worry about hurting people’s feelings.

Yet Mr. Sussman wants to protect Mitford’s correspondents, to mitigate their pain, and is even willing to hide the identities of Communist Party members who have not, in his words, “outed” themselves. I cannot believe, based on the evidence of the very letters that Mr. Sussman provides, that Jessica Mitford would find his concerns about the tender feelings and reputations of others worthy of respect.

Mr. Sussman’s motives are all the more suspect since Mitford’s own Communist Party membership is one of the least attractive features of her biography. While the wayward Mitford was a problem for the party, since she was by both nature and nurture such an independent soul, she nevertheless lent her talents to an undemocratic and conspiratorial organization that took its orders from a foreign power.

Why? Because for her the party stood for social justice, especially civil rights, a laudable concern Mitford championed in countless ways in the San Francisco Bay area. She not only wrote about social issues, she put her day-to-day energies into the drive for equal rights.

At the same time, Mitford, who prided herself on her investigative skills, turned a blind eye to the global and geopolitical actions of the party, headquartered in Moscow. Take, for example, the astonishing letter Mitford wrote after visiting Hungary shortly before the 1956 rising, which (Mr. Sussman notes) resulted in 30,000 deaths in Budapest alone: “Why couldn’t we see signs of this while we were there?” she wrote to her mother-in-law. Why indeed. Anyone who traveled, as I did, in communist Europe right through the end of the 1970s, had to be aware of repressive and closed societies that produced sullen functionaries and a cowed populace ready to unburden itself to visiting Americans if an appropriately secure location could be arranged.

Mitford (a member of the party until 1958) expressed some sympathy with the uprising, but look how she frames her discussion:

However, I gather from news releases that the rebels were quickly joined by fascists and that a “white terror” was being established. Because of this, I think in the long run the interests of the Hungarian people are best served by entry of Russian troops.

It takes your breath away. As Woody Allen said in “Annie Hall”: “Excuse me, I’m due back on planet earth.”

While Mitford’s criticisms of American social institutions often hit their mark, the Soviet Union, until very late in the day, got no more than wry wrist slaps. Her letters disclose a love of causes, and even the fawning Mr. Sussman admits that sometimes Mitford was not doing much more than stirring the pot.

Mitford was a muckraker, and the downside of a continual raking of the muck is that the raker can get pretty soiled herself. I was dismayed to read, for example, these gloating passages:

I was also successful in getting the book [a history of the Mitfords written from a pro-Diana bias] thoroughly trashed by reviewers in S.F. Chronicle, Boston Globe, & NYT Book Review, having pointed out to reviewers — all friends of mine — some of the stupider passages.

More on OJ: Bob [Treuhaft, her husband] & I rather agreed with you … we were pleased with verdict but thought he’s prob. guilty … serves the cops right. A thought: sort of an Affirmative Action type of vote? Redressing centuries of injustice in our law courts?

Forget the brutality of the murder and find your solace in revolutionary justice. No wonder it took Mitford so long to leave the communist party. Elsewhere she writes that she supported Stalin: “Mainly for pragmatic reason that his lot won. Trots lost. I think Trotsky wld have been more to our liking philosophically.” Her inhumanity is striking.

For all the good Mitford did in exposing corruption, there was corruption at her very core as well. She seemed to have little understanding or empathy for liberals like Senator Clinton, who once was an intern in Bob Treuhaft’s law firm, but later helped her husband reform the penal system in Arkansas.

Rebecca West, who in some ways had a temperament similar to Jessica Mitford’s but drew very different political conclusions from her investigative reporting, might have said Mitford lacked a sense of process, a grasp of the mechanisms by which genuine social change is accomplished. Mitford was curious about West and wrote to me, wishing to know more about my research for a biography of West.Too bad Mitford did not take to heart West’s key insight: That no matter how slow and contradictory it might be, there is no substitute for the Rule of Law — a phrase West liked to capitalize. Resort to revolutionary justice results in no justice at all.

crollyson@nysun.com