It’s Not Literature, but It Is Good Fun : JUST DESSERTS, <i> by Patti Massman and Susan Rosser,</i> Crown, $18.95, 336 pages

This is a weird but wonderful idea: A diet plan buried in a novel. “Just Desserts” is a “novel of ideas” as surely as any literary narrative by Diane Johnson or Joyce Carol Oates. The differences are there, however, and they are crucial.

Johnson may write about the essential invisibility of a woman in a man’s world; Oates may insist that violence is a primary component of human nature. But Patti Massman and Susan Rosser take a more practical approach to things: By limiting the amount of fats in your diet to under 30%, you can lose a scandalous amount of weight, and--more abstractly--in any divorce, forget the kids, the child support, the house, the car, the money, even the friends. The rule of life is: Whoever gets custody of the most of fun wins.

Make no mistake: This novel is very badly written. (But then, Theodore Dreiser was a clunky writer, and the force of ideas carried him through.) The ideas are what shine here, and the beautifully evoked sense of place, which in this case happens to be the San Fernando Valley, south of Ventura Boulevard.

Diana Lowe is a housewife, a Size 16 housewife. Her children worry about her and are embarrassed by her. Her mother, a wonderful woman who loves to joke and eat and bake, is up for a heart attack--which she sustains in Chapter 1. Diana has girlfriends with moderate eating habits and a wonderful neighbor, Christopher Berry, a Hollywood writer who often drops by for small talk and snacks.

Diana is married to a Husband from Hell. (Again, the authors’ intimate knowledge of Los Angeles types is what carries this characterization.) Harvey Lowe is the one thing worse than a self-important, narcissistic, arrogant, needlessly cruel, self-regarding dentist. He’s an oral surgeon. With a hair transplant.

Harvey loathes every fat cell on Diana’s body. (The truth is, he’s the kind of person who loathes everyone; a creepy perfectionist and the kind of spouse who delights in being mean to his wife the way a dog-kicker loves to kick dogs, or certain dentists drool in anticipation when they pick up their drills.) Harvey is a bully. He’s mean , and he soon leaves Diana to take up with his svelte, gold-digging dental assistant.

Heartbroken, Diana visits a physician who tells her not to worry about losing weight but to begin getting this fat out of her bloodstream. Diana is allergic to fats, OK? A droning (but interesting) lecture sneaks into the text here about how four grams of fat can make you as fat as nine grams of carbohydrates. . . .

Harvey finagles their finances so that he gets all the money. He cuts off his wife’s credit cards and cleans out their bank account. He manipulates their relationship so that he ends up with the house.

But while subsisting on a diet of complex carbohydrates and buying a nice supply of yellow eye shadow, Diana loses weight, meets new friends, strengthens her relationships with her children and gets a free trip to Switzerland, where she meets multimillionaires and has wonderful sex with her masseur.

She becomes a celebrity expert on weight loss and wears designer clothes. She even lectures on weight to the girls at Stony Ridge Academy where this diet plan--the less fat the better, but sweets and Champagne are OK--is spelled out in considerable detail.

Of course, Diana finds a wonderful man who loves her whether she’s fat or not, a nice person , which, in answer to Dr. Freud’s perennial question, is what women really want. As for that odious spouse who spends his professional life with his fists in other people’s mouths and his personal life minding other people’s business, his hair transplant gets all weedy, he develops a paunch, and his second wife--that dental assistant--gets pregnant with twins.

Not literature, but there are enough subversive ideas here to wreck the butter industry, dent the dental profession and raise the sales of Champagne.

Next: Constance Casey reviews “The Pulitzer Prize” by Douglas Bates (Birch Lane Press).