Dragon Slayer of the Gender Wars : FICTION : WORST FEARS,<i> By Fay Weldon (Atlantic Monthly Press: $21; 200 pp.)</i>

In the war between the sexes--which in this day is more like a Korean police action, undeclared but endlessly seething--Fay Weldon is the unregenerate guerrilla fighter, Ms. No Quarter Given, Ms. “What Geneva Convention?”

A hundred years hence, if people can still read, her books will likely have the unblunted edge of Jane Austen, unsentimental Baedeker guides to sexual manners in an ill-mannered age.

The best known of Weldon’s score of books is “The Life and Loves of a She-Devil,” a poison-pen chain letter, its eponymous heroine a Count of Monte Cristo with breasts, taking her revenge like an epicure--cold. Naturally, the Hollywood movie machine reduced its lapidary malice to slapstick and cat fights.

Now comes “Worst Fears.” While his actress wife, Alexandra, is in London, playing Nora in Ibsen’s “A Doll’s House,” drama critic Ned Ludd dies of a heart attack in their 900-year-old English cottage. It is an engaging launch to a plot that peels away character like Russian nesting dolls, one self within another, one self concealed by another.

From the moment Alexandra’s friends visit Ned’s body on the morgue slab, lifting the cloth to peek at his penis, his death becomes a pulled thread through the unwashed fabric of manifold lives.

At first, bereavement sends Alexandra’s mind telescoping in and out, stone-skipping from the trivial to the eternal, from calculating how often they had sex in 12 years of marriage to imagining 20 years hence when his books will be on secondhand shelves and his name forgotten.

While Alexandra is a woman unobservant of nuances, focused, self-absorbed--in short, rather male--even she cannot, at last, help but wonder, even as she resists wondering, what happened.

What had he been doing to die of a heart attack at 49, with “Casablanca” on the video? Who was the fat, ugly woman keening in her garden and then making herself at home in Alexandra’s kitchen? Why had her friends so thoroughly cleaned up the place before Alexandra returned?

And the sentimental solace of counting their couplings disappears as she does so. “What did she know?” Alexandra asks herself with more truth than melodrama. “She was only the wife.”

It’s a fine setup for a murder mystery but an even better one for a murder of the heart. Her marriage turns out to have been a series of flats and props, a charade all the more wretched for her husband’s doubtful taste in other women, notably Jenny Linden, whose very slovenliness makes Alexandra feel “so shop soiled, so picked over.”

The noble widow breaks into Jenny’s house, steals her diary, finds that Jenny has talismanically appropriated Ned’s toothbrush and his deathbed socks, and that Jenny--who earns a living making set and costume models--has made, voodoo-fashion, a model of the Ludds’ bedroom, down to the ancient brass bedstead where Ned died in flagrante.

With discovery comes detachment, a quality Weldon’s heroines wield like a shield and buckler. Even Alexandra’s brother-in-law hectors her to be more understanding of Jenny because “it’s hard for women when their married lovers die.” Ultimately it is Jenny, scarlet-clad and hysterical at the funeral, whose photo appears in the newspaper, captioned as the grieving widow, while the grieving widow is in London interviewing for a part in a Hollywood film.

In her own make-believe, Alexandra frees herself from both the real past and the imagined one. “In the end,” she realizes, “you didn’t want the truth, you just wanted to know what happened.” (If anything, Weldon is overfond of scorched-earth denouements, torching the scene of the crime, Manderley-like, in an emotional suttee that consumes everything but the widow.)

Weldon rewards her readers with characters like Vilna, the one female neighbor who (as best I can figure) didn’t sleep with Ned. Her Yugoslav bombshell looks are fading because her husband is in prison for some financial scam, depriving her of her miracle skin treatment--semen. “One can be so wrong about people” she tells Alexandra. “Even if married to them. I lived with Clive for four years, and never knew he was a crim.”

Lay one of Weldon’s modestly sized books on a scale opposite one of those ponderous, pink and gold-covered three-name lady author novels that fill the racks at airports and the little Weldon opus will tip the scales, Essence of Verity against goopy, souffleed mail-order catalog prose.

Perhaps the closest that American writers have come to this clear-eyed fearlessness was the acidulous and bibulous Dorothy Parker, or Anita Loos, whom H. L. Mencken once told in envy and dismay that she had broken the American taboo of making fun of sex. Fay Weldon breaks taboos like the tape at a marathon, and she hasn’t stopped running yet.