AARON GUNNER, the black Los Angeles private eye, reluctantly takes on a client whom he positively detests -- a foul-mouthed, cocky, sneering "gangbuster," a member of the street gangs that terrorize the poor neighborhoods of the city.

"I don't see why I should have the slightest interest in what happens to your client, counselor . . . {and} get his act back out on the street -- my street, remember," Gunner tells the public defender who wants to hire him to help her defense of a gang member accused of murder. But he finds he can't turn down this determined young woman who believes her client -- obnoxious as he may be as a human being -- has been framed.

Gunner, the soft-hearted tough guy (aren't all private eyes?), is back for his second adventure in Not Long for This World (St. Martin's, $17.95). His debut in Fear of the Dark won 1987's Best First Private Eye Novel contest for author Gar Anthony Haywood.

Against his better judgment, Gunner agrees to try to find evidence to clear Toby Mills, accused of being the trigger man in the drive-by slaying of Darrel Lovejoy, who had escaped the ghetto and then returned to organize a volunteer peace patrol to defuse gang-related violence. Lovejoy was good at his job of human salvage, trying to rescue street hoodlums, who are "not long for this world" because they lead violent lives of crime and drugs.

A witness has identified Toby and another member of the Imperial Blues gang as the hit team that shot down Lovejoy. Gunner's only lead is to track down the elusive Rookie Davidson, the driver. He learns that it is a dangerous game to try to get information from gangbusters, who don't like inquisitive outsiders prying into their secret alliances and the locations of their arms "cribs."

As much as he abhors his client, Gunner turns up evidence that Toby may have been framed for Lovejoy's murder. The witness at the bus stop turns out to be a crack addict who trades testimony for a month's supply of free drugs.

Gunner's search for Rookie leads to Rookie's older brother, Teddy, a successful businessman, who has pressured Rookie to quit the gang. There also is a charismatic minister who had been working with the Lovejoy to bring the gangs together for a "peace summit." And has there been a hit list out on the gang members themselves?

Haywood, who has lived in Los Angeles all his life, shows us he has a sharp eye and ear in his depiction of the world of the street gangs and the mean streets of Los Angeles. The fear and hopelessness are palpable.

Other writers also have staked out the same territory. It's an unpleasant scene that can get downright depressing if visited too often. The monosyllabic street jargon, colorful and and true as it may be, doesn't lead to sparkling dialogue or great insight. Haywood still must prove that he has a distinctive voice and that Gunner is more than just another L.A. private eye. Death on the Farm RALPH McINERNY, who writes the delightful Father Dowling mysteries (now translated to television with the addition of a thoroughly modern nun who can shoot a hoop or deal blackjack), is embarked on a second series, this one featuring a small-town Indiana lawyer. Andrew Broom makes his third appearance in Savings and Loam (Atheneum, $17.95) -- in what proves to be more a cat-and-mouse thriller that a puzzling mystery.

From the first chapter, we know who gunned down the new owner of the Krueger farm. What the killer does not know is that someone has heard the shots and watched as he ditched a duffel bag into the river to get rid of the World War II M-1 gun. The duffel bag soon is fished out of the water by Leroy White, a balding, middle-aged family dropout who has run off with a sexy young gal to escape a Dairy Queen franchise and a wife and kids who show him no respect.

It is Leroy, the overage hippie, who is the fulcrum of the narrative action in Savings and Loam. His attempt to blackmail the killer is to lead to tragic consequences that turn him into a man bent on revenge. Leroy becomes a hunter stalking the killer while the police are baffled by the apparently motiveless shooting on the Krueger farm.

It remains for Broom, who gets involved because he has handled the sale of the farm property, to pull the pieces together, although his role is almost peripheral in Savings and Loam.

All the small-town characters are well-drawn individuals. Broom is a quiet, reassuring man, content with his decision to shun big-city Chicago law to return to his hometown. Leroy transcends the stereotype of the overage hippie to enlist sympathy as a man who has worked hard to raise a family and run a business and now finds "how worthless all his efforts were" while he thought he was gaining credit, "the way they gave you stick-on stars at the library for reading during vacation when you were a kid."

McInerny is a tantalizing storyteller who weaves together the separate narrative threads and builds suspense to a slam-bang shootout in which Leroy may or may not have followed Broom's advice to "let the law do it."The Judge's Alibi WITH A wealthy and prominent clientele, Brady Coyne, the Boston lawyer-amateur sleuth, knows he must protect his reputation for discretion and jealously guard the lawyer-client realtionship. In Client Privilege (Delacorte, $16.95), Coyne must ask a terrible question: Has one of his oldest and best friends -- a judge -- committed a murder and set up Brady to take the fall, cleverly manipulating his commitment to the principle of client privilege?

In this ninth in the solid series by William G. Tapply, the judge asks Coyne to keep a rendezvous with a blackmailer threatening to reveal an extramarital affair in the long-ago past. That hardly seems grounds for blackmail these days, and Coyne angrily rejects the blackmailer's threat in the neighborhood bar where they meet.

That same night a television investigative reporter is found dead, and Coyne recognizes that this is the same man -- disguised with mustache and dark glasses -- he met in the bar. Coyne soon is the prime suspect in the murder after he pleads client privilege to protect the judge's name when police question him. Yet Coyne has nagging doubts about his friend, who may be less than honorable, when he discovers the secret from the past involves more than an innocent affair.

Tapply is not in top form in Client Privilege. He pads out a skimpy plot with too much talk as the police repeatedly question Coyne and he does much soul-searching about a lawyer's role in a system that works sometimes but not always.Widow's Walk IT IS A delight to welcome back Clara Gamadge, a sprightly 70-year-old with wit and warmth, as she returns in Murder Observed (Henry Holt, $15.95). Once again she proves a charming companion as amateur sleuth-narrator.

Last year Eleanor Boylan, the niece of the late Elizabeth Daly, who wrote the civilized Henry Gamadge mysteries in the 1940s, decided to revive the series with Henry's widow, Clara, carrying on the family tradition of sleuthing. In Working Murder, Clara's debut, Boylan, an author of short stories in her own right, proved that she has her aunt's touch with a murder mystery fashioned in the traditional British "cozy" style.

This time Clara is watching from her rain-splattered apartment window as Anna Pitman, one of her oldest friends, tries to hail a cab. A skidding car strikes Anna, and the police list her death as an unfortunate traffic accident. But Clara has seen the driver's face and knows Anna was deliberately run down, even if she doesn't know a motive.

Clara, with the support of her son, daughter-in-law and visiting cousin from Florida, must sort out a tangle of clues including a missing letter and an old photo from the 1936 Olympics. The stalwart Clara traps a murderer and then attends an old friend's wedding with equal equanimity.

Jean M. White regularly reviews mysteries for Book World.