Columnist and children's book author Viorst infuses her premiere adult novel with the quicksilver pace, naughty optimism, and heartwarming humor of her bestselling nonfiction (Necessary Losses, 1986, etc.). The novel's premise is wacky: Brenda Kovner, the narrator, 46- -an accomplished hostess, gourmet cook, devoted wife of Jake, a pediatric surgeon, mother of their two grown sons, Jeff and Wally, and a wildly popular nationally syndicated advice columnist- -resolves to murder a somewhat sinister D.C. neighbor named Mr. Monti, who is ``doing harm to my family.'' The reason he's doing harm: His adored youngest daughter Josephine wants to marry Wally, who refuses to convert from Judaism to Catholicism for the occasion. The harm he's doing: As Brenda doggedly discovers, he's been bankrolling a number of baseless malpractice suits against Jake, is laying a plan to entrap greedy elder son Jeff in a bogus real-estate deal for the purpose of bankrupting him, and seems to be planning to have Wally killed on Halloween. Brenda--who's had sex with Mr. Monti once as the third stage of a one-day experiment in the pleasures of adultery after a lifetime if monogamy--first tries to poison him, then nails him (or rather, by mistake, his twin brother) into an airtight closet, and finally hires two muggers she's met while investigating Jeff's real-estate troubles to assassinate him. Nothing works--luckily for Brenda, since it turns out that (a) a psychotic social-work client is the one who's really been trying to kill Wally, and (b) Mr. Monti's wife controls all the family's assets and has no intention of letting Mr. Monti waste them on tormenting the Kovners. Finally, Wally and Josephine- -who's gone through a lesbian phase--marry; Brenda and Jake, whose marriage has been rocky, to say the least, reunite. Facile, funny--Viorst at her best.