BOOK REVIEW / FICTION : Sex, Merriment and Thoughts of Murder : MURDERING MR. MONTI: A Merry Little Tale of Sex and Violence <i> by Judith Viorst</i> ; Simon & Schuster, $21, 254 pages

M erry is the operative word here, although sex and violence do figure into the equation. Even so, fans of Chandler, Hammett and company may find Judith Viorst a tad bland in the department, though she rates at least a six for sex.

“Murdering Mr. Monti” is actually about thinking of murdering Mr. Monti, an exercise that provides ample opportunities for the author’s wit and antic humor, but offers relatively few chances for overt antisocial acts.

I’m not giving anything away if I tell you that a successful advice columnist with two grown sons and a pediatric-surgeon husband is not your typical killer. A taut 46, Brenda Kovner is famous for her pot roast; loyal, kind, somewhat over-maternal and extremely well-organized. She’s proud of her “can-do” attitude, which infuses her thousands of readers with confidence. On her good days, Brenda resembles Goldie Hawn, a fact bound to make the casting of this movie easier.

The 46th birthday is crucial to the plot, because as the day approaches, Brenda decides that she’s been deprived of sexual variety, having married at 20 and committed herself to marital fidelity.

Initially saddened by this fact, she realizes that it is not immutable. She can change it merely by going to bed with a few other men, each carefully chosen to combine as many unknown qualities as possible. Ideally, she’d like to sleep with a political radical, with a man younger than her husband, Jake, a man of a different race, one of a different religion, a genius, a celebrity, and an identical twin.

To her delight, (because Brenda has no desire to make a habit of adultery or become unnecessarily promiscuous), she finds three men in her circle of acquaintances who possess several of the desirable traits. One qualifies on the basis of race, youth and political activism; a TV personality fills the bill for fame and genius; and the third candidate is Mr. Monti, who makes the short list because he’s a twin, married and of another spiritual persuasion.

Though Brenda is in no particular hurry to fill the gaps in her store of carnal knowledge, she’s able to complete the entire project in one extremely hectic 24-hour period. Treated with considerable levity, these encounters live up to expectations aroused by the subtitle. They also embolden our heroine to believe that if she can commit undetected adultery, murder shouldn’t be any more difficult.

Mr. Monti is the only man short-listed for murder. He is no disappointment as a lover, but none of his other qualities is in the least endearing. He’s vulgar, prejudiced, fundamentally sexist and adamantly opposed to the marriage of his daughter Josephine to Brenda’s son Wally. Moreover, he wears a pinkie ring.

Murder would never have occurred to Brenda if Mr. Monti had not persuaded her other son, Jeff, to invest all of his worldly assets in an extremely dubious urban-renewal project. When the project goes down the tubes, Mr. Monti calls in his markers, which include Jeff’s entire net worth and then some. Mr. Monti has lent Jeff the seed capital, and now wishes to concentrate upon cultivating his own financial garden. Various menacing phone calls, notes and scary incidents convince Brenda that her son’s very life is in danger from Mr. Monti’s thugs, and her solution is to neutralize the man she believes is causing the trouble.

Pleading and persuasion have failed, and while Brenda believes Jeff has made a mistake, paying with his life seems excessive. This is where the overly maternal part kicks in. Most mothers would think bank loan or lawsuit, but Brenda thinks murder. “I can handle this” is the rule she lives by.

Handling it proves to be as much fun for the reader as it must have been for the writer. Though the results are predictable, the methods Brenda chooses are highly ingenious, demonstrating that you can’t really learn how to kill from watching old movies on the subject, because up until the mid-’60s social codes in fiction, film and TV dictated that the guilty be punished.

Brenda’s hottest tip comes from a pair of funeral directors who sit behind her on a plane, but events conspire to thwart her plan. In the end, that turns out to be the best thing that could have happened, inspiring Brenda to an even more imaginative solution, and a satisfactory, if somewhat sentimental, finale. You might enjoy the merriment and the sex enough to forget you were promised violence and murder.