She Had Every Reason to Do It : AFTER ALL THESE YEARS, <i> By Susan Isaacs (HarperCollins: $23; 343 pp.)</i>

Susan Isaacs writes wonderful books. They’re loaded with wit, crammed with memorable characters, endlessly entertaining and beautifully written. And they’re not half bad in the plot department, either.

“After All These Years” revisits one of Isaacs’s favorite storylines: Everygal in a heck of a jam. Rosie Meyers, a high school English teacher, wakes up in the middle of the night to discover her almost ex-husband on the kitchen floor with a carving knife in his chest. In a panic, she gets her fingerprints all over the murder weapon and quickly lands in deep trouble.

Rosie’s a logical suspect, with ample motive. Richie Meyers became fabulously wealthy almost overnight through his business information service and promptly moved to a house on Long Island Sound pretentious enough to have a name, a library featuring Books by the Yard, and furniture upholstered in fabric “custom-dyed in vats of tea so it would look yellowed, slightly shabby.” Embarking on a colossal mid-life crisis, Richie began calling himself Rick and then announced--the morning after their silver anniversary gala attended by 200--that he was leaving Rosie. Men have been killed for far less.

Richie/Rick had a replacement for Rosie all lined up, his senior VP, Jessica Stevenson. Jessica is 38 and seasoned, a woman who in a business crisis “trotted out all the old pre-feminist tricks--the southern belle’s we-have-a-secret smile, the call girl’s stroking of her neck and calves, the teen-ager’s toying with her hair--that no liberated woman would dare use in a business situation.”

Rosie knows she didn’t kill Richie, and when the police settle on her as their prime suspect, she decides to find the killer herself. Time is running out; she’s to be arrested soon after Richie’s funeral, held at a mortuary “whose exterior seemed to be modeled after Monticello--if Jefferson had a cousin in the aluminum siding business.”

It’s self-determination time. Late one night, Rosie descends a Sav-Ur-Self metal ladder in ridiculously impractical designer tweed slacks and a cashmere sweater and heads for Manhattan to investigate.

She makes plenty of initial mistakes. At 47, she’s unaccustomed to life on the streets, much less being subject of a $50,000 reward. She forces confrontations, loses all her money and finds herself reduced to stealing a stranger’s fast food burger.

She discovers, however, that she’s got some unconventional resources, and she uses them wisely: one of her son’s friends, who has become helpfully felonious; the disgruntled genius cut out of Richie’s business; Richie’s loyal old secretary who got pinkslipped at the same time Rosie did; a high school boyfriend turned business tycoon.

The victim is often the most interesting character in a mystery. That’s not really the case here. By all accounts, Richie/Rick was a real piece of work in the sack, but any other attributes get lost early on. It’s a little hard, actually, to figure why Rosie wanted him back.

In her investigation, Rosie overturns a lot of rocks, and many reveal nasty surprises. Richie’s barely-bereaved fiancee Jessica, now acting CEO of his company, has been seeing an infinitely more wealthy and powerful man. Richie himself was a serial adulterer. His business may be about to go public. And so on. Throughout it all, Rosie retains a humorous fatalism, an offhand flippancy that’s occasionally tiresome but generally amusing.

Susan Isaacs is more than just a talented novelist. She’s also a cultural anthropologist, specializing in the Long Island upper crust who populate her home turf.

Here she assembles a marvelous cast of unforgettable characters: A bland and humorless allergist called Suspicious Foods behind her back. A beautiful neighbor, once a litigation associate at “one of those gigantic New York law firms that, with great pride and fervor, defend petrochemical conglomerates in lawsuits brought by cancer-riddled former employees,” now the quintessential over-accomplished perfectionist supermom.

Under certain circumstances, the fugitive state can be inherently appealing, almost romantic, as witness Richard Kimball (“The Fugitive”) and Patty Hearst. Last year’s “Blanche on the Lam” by Barbara Neely approached this situation from the opposite end of the socioeconomic spectrum, with an African-American cleaning lady disappearing into the domestic underground. Like Blanche, Rosie Meyers survives and triumphs because she cleverly and resourcefully plays the hand she’s dealt.

Beginning with “Compromising Positions” in 1979, Susan Isaacs has written half a dozen highly entertaining novels. “After All These Years” could stand on its own as a credible mystery, but it’s more than that. It seriously examines the plight of the discarded wife. More to the point, however, it’s pure fun and perfectly timed for summer reading.