RARELY DOES a first mystery novel meet with such critical and popular acclaim as that which greeted Patricia Cornwell's Postmortem when it was published last year. Now comes Body of Evidence (Scribners, $18.95), and we know that Cornwell is not a one-book talent. Postmortem was, happily, no fluke.

In her second mystery, Cornwell again takes the reader into the fascinating world of the forensic crime lab, where a single fiber picked up under laser can provide a vital clue. This time she ventures beyond the lab to explore complicated human relationships and the personal life and emotions of her heroine, physician Kay Scarpetta, the state of Virginia's chief medical examiner.

A lost love from college reappears across a void of 15 years soon after Kay has been called to a bloody crime scene to collect forensic evidence in the death of Beryl Madison, a successful writer who has been brutally slashed to death. Why, Kay must ask again and again, did Beryl, who had fled to Key West to escape death threats, turn off her burglar alarm and open the door to her murderer on the first night of her return to Richmond?

With their high-tech forensic equipment, Kay and her co-workers discover samples of 10 different fibers left by the killer, including one with a strange three-leaf clover configuration. The case has puzzling aspects beyond the forensic discoveries. Beryl was in a contract dispute with her former mentor, Cary Harper, who burned out after writing one Pulitzer prize-winning novel. Mark James, Kay's love from the past, may be linked to an unsavory lawyer who accuses Kay of stealing a lost manuscript rumored to be an autobiographical expose by Beryl.

Kay follows leads that take her to a Key West bar, a car wash, a retirement home, a psychiatric hospital and the old Virgina estate where Harper lived with his sister and Beryl, who came there as a promising 16-year-old.

Body of Evidence is a complex, multi-layered novel. With enough twists and turns for two books, the plot is overburdened but Cornwell manages to sustain the narrative energy. Although she generally handles human relationships with sensitivity and insight, she fails to spark the romantic attraction between Kay and Mark. There doesn't seem to be much chemistry there.

Cornwell concedes that Kay is "pro-active" as a medical examiner and exceeds the normal bounds of her profession in investigating homicides. How fortunate we are that she does! The Semi-Private Eye THE WARNING to back off comes from an arrogant, private-school punk: "The Sebastian story is complex, Mr. Semi-private Eye. It's straight out of Kafka -- nothing is as it seems; no one is unsullied; guilt and innocence are indeterminable."

"It sounds more like Ross Macdonald," is the rejoinder from John Marshall Tanner, San Francisco lawyer turned private eye.

And it also sounds like Stephen Greenleaf, who has been hailed, more than once in the last dozen years, as the anointed successor to carry on the tradition of the Raymond Chandler-Ross Macdonald school of the hard-boiled crime novel. In Book Case (Morrow, $18.95), Greenleaf again demonstrates that he can fashion as good a traditional private-eye novel as anyone writing today. The problem is that new writers seeking their own voices have pushed beyond the bounds of that formula to explore new areas. Greenleaf does not break new ground, perhaps because he is too traditional. Still, his is not an accomplishment to be undervalued.

In Book Case, Tanner is asked to find the author of a manuscript sent anonymously to a small San Francisco publishing house. Periwinkle Press is owned by Bryce Chatterton, an old friend who wants to publish the manuscript because he believes it can become a best seller. Soon fact and fiction begin to blur as Tanner tries to trace the author of this emotional story of an idealistic teacher falsely accused of raping a student.

Although names are disguised in the manuscript, there are hints that point to the exclusive private Sebastian School, and Tanner's research turns up a parallel case involving a teacher there a decade earlier. The teacher, Wade Linton, pleaded guilty to a lesser charge after being accused of rape. He served a prison term and has been paroled. The last chapters of the manuscript sent to Chatterton indicate Linton may be seeking revenge for being framed.

Greenleaf is a careful and meticulous craftsman: There have been only a half-dozen Tanner novels since his first appearance in Grave Error in l979 (others include Death Bed and Toll Call). Greenleaf's plots -- much like those of Macdonald -- often begin with a missing person and lead to a search of the past to find the truth behind lies and deceptions. There usually are complicated family relationships and childhood traumas.

Tanner, as Philip Marlowe and Lew Archer before him, is a loner, or, in his own words, "more a recluse than a celebrant." Scratch his skepticism and you will find a man with his own brand of idealism. One nice thing about Tanner is that he is not the cliched, world-weary cynic. He seems reasonably content with his life. He wouldn't ask for more.

With all the buried secrets from the past, Greenleaf has to wind up with a summary to sort out the entanglements. The identity of the author of the manuscript is only a mild surprise. In the end, no one is left unsullied, and guilt and innocence are indeterminable.The Prime Plant IF IT'S Martha Grimes, there must be a British pub in the story and title. This time it's The Old Contemptibles (Little, Brown, $19.95), which again offers both the charms and annoying excesses of this popular series, which achieved the best-seller list with last year's entry, The Old Silent. Grimes has cultivated a devoted band of followers who value style more than plot substance.

In this latest Grimes, Scotland Yard Supt. Richard Jury himself becomes a suspect when Jane Holdsworth, with whom he had had a brief, passionate affair that had progressed to his purchase of a ring, is found dead of a barbiturate overdose. Suspended from duty, Jury has to dispatch his aristocratic friend, Melrose Plant, to investigate the Holdsworth family.

Poor Melrose. He must leave behind his beloved Bentley and custom-tailored clothes as he goes undercover as a cataloguer-librarian for Crabbe Holdsworth, a student of the Lake poets who dismisses Wordsworth and Coleridge in his ardent admiration for Robert Southey.

The Holdsworth family household seems a very unlucky one, indeed. Within six years, there have been a series of deaths -- two apparent suicides and two "accidental" falls on the rugged Lake District terrain. The victims have included Crabbe's first wife; his son (Jane's husband), found hanged, and a woman who was the family cook. Now there is Jane, apparently a suicide like her husband. Yet there are enough discrepancies to raise the question of murder.

As in all the Grimes mysteries, there is a clutch of quirky characters to enliven the scene: 89-year-old Adam, the family patriarch; his great-grandson, 16-year-old Alex, who has worked out a way to beat the betting parlors, and Lady Cray, a wealthy, elegant kleptomaniac, who is Adam's friend at a posh retirement home. And there is Millie, the 12-year-old daughter of the cook who died in a fall, whose relationship with Alex is genuinely affecting.

The Old Contemptibles is the 11th book in as many years since Grimes started the series in 1979 with The Man With a Load of Mischief. Grimes dotes on English eccentrics to a fault and is more than believably British. Melrose Plant's bloodline can be traced to aristocratic amateur sleuths like Lord Peter Wimsey and Albert Campion. Jury is the affable, urbane Scotland Yard detective in the mold of Roderick Alleyn. There are times, however, when an irritating coyness creeps into the narrative when Grimes is trying to be so cleverly arch.

Yet the charms of Grimes's books are not to be denied. Her prose is stylish and elegant, and she has a Dickensian touch in her scenes of rollicking farce (it's great fun when the residents of the retirement home upset the well-ordered routine). She brings a sensitive understanding to her portrayal of children, and they are always memorable characters.

Though Grimes has described her plots as "often idiotic," she is too harsh on herself with such strong language. Yet it is true that the plots often seem just a device on which to hang witty and ironic observations. In The Old Contemptibles, the plot is a jumble, cluttered as it is with discursions. The identity of the culprit is contrived, and the motivation is far from credible. What undoubtedly was designed as a shattering denouement seems staged and implausible. For those who are devoted fans, however, these faults are more than offset by Grimes's stylish use of language, her wit and her characterization.W Jean M. White regularly reviews mysteries for Book World.