The Eighth Commandment by Lawrence Sanders (Putnam’s: $18.95; 384 pp.)

The resort industry ought to give Lawrence Sanders an award, maybe a Lucite-encased sand castle splattered with Technicolor blood. Year after year, practically without fail, Sanders has been turning out the perfect American summer leisure article, a novel composed of mystery, mayhem and biblically scaled immorality, always with a plot heavy enough to keep vacationers reading and light enough to withstand inevitable interruptions by howling children, wayward pets, overheated ice cream salesmen and disappearing American Express checks.

This year’s entry is “The Eighth Commandment,” the not-too-implausible story of a female beanpole from Des Moines who moves to New York, becomes a numismatist and, in the course of this novel, a private eye. Of course, she beats the pros of the police and the insurance company by solving the theft of a Demaretion, a silver dekadrachm of ancient Greek mintage. The coin is lifted from the impeccable and fabulously valuable collection of Archibald Havistock, one of those Manhattanites who seem to exist only in novels requiring wealthy, urbane ciphers. Naturally, the theft is the fuse to a series of murders, enough of them to make dead bodies a major form of interior decoration in those fancy town houses and condos.

How Mary Lou Bateson, nearly 6 feet 2 inches tall and nicknamed “Dunk” for her skill with a basketball, rises from the rare-coin department of an auction house to the sex, drugs and rock ‘n’ roll of New York’s rich underbelly is fairly predictable. But Sanders has given her the grace to do it ungracefully. Along the way to pinning the crime on the unlikely perpetrator--while clearing herself--Bateson overdoses on Milky Way candy bars and sleeps with both the cop and the insurance investigator. Thankfully, she does not worry about contracting any of the cornucopia of sexual diseases. She sometimes worries about gaining weight, though.

In fact, food seems to have taken the place of sex in this novel. As in other Sanders books, especially “The Case of Lucy Bending,” there’s a fair share of bedroom frolicking and passion--not to mention the kinky couple and their many friends who videotape their pretzel imitations on the living room rug. But these scenes are often routine and without energy. Sanders seems to have gotten the 1980s down pat--food is more interesting than sex. Orgasms are OK, but Big Macs and pasta really catch the characters’ attention.

Despite its weaknesses--including the impression that Sanders is only going through the motions in some chapters--”The Eighth Commandment” is just what the cover blurb says it is--a diverting amusement that does not take itself too seriously. And Mary Lou Bateson, outlandish as she seems at first, is one of the more engaging characters to come out of Sanders’ menagerie of oddballs and maniacs. She is healthy, sane, friendly, loyal and as down-to-earth as Iowa. With any luck, she’ll be back.