OTHERS write police procedurals. Nat Hentoff writes street procedurals.

While other writers tell us about cops and how they handle their cases, Hentoff takes us down the mean streets of New York's Lower East Side and tells us about the people and how they live and survive -- the junkies, pimps, hookers, drug dealers, survivor-citizens who run the delis and newsstands, street- smart kids, and cops themselves.

The Man From Internal Affairs (Mysterious Press, $15.95) teems with the sights, sounds, and smells of life on the seamy underside of the naked city. Some characters are not pleasant. Others are too earnestly funky or lovably eccentric. But all come alive.

Hentoff does make a perfunctory bow to the standard police procedural. He has a cop and a case. His central character is Noah Green, the middle-aged, jazz-loving police detective who made his debut in Blues for Charlie Darwin three years ago. Noah, in his early fifties, is now married to the younger Shannon, a reporter, and has a baby son. Somehow he has managed to survive as a decent human being after nearly three decades in a tough precinct.

Noah is assigned to investigate a series of grisly murders (half-corpses, severed at the waist, are being stuffed in garbage cans). "Blondie Thrown Out With the Garbage" screams a headline in the New York Post.

At the same time that he is trying to track down the butcher-killer, Green is under surveillance by the Internal Affairs Department. An old pro like Noah quickly knows when he is being tailed. But why? Innocent acts, like his monthly lunch with a high-school chum who now is a criminal lawyer, take on suspicion after the IAD receives an anonymous letter warning that Noah is on the take.

Almost casually, Hentoff reveas the identities of the garbage-can murderer and the anonymous letter-writer less than two-thirds of the way through the book. There is little mystery or suspense in The Man From Internal Affairs. What Hentoff does offer is a ripe assortment of cameo characters and colorful street vignettes spiced with black comedy and irony.

There is Angel, the savvy Puerto Rican teen-ager who turns down an offer to join the local gang because he wants "to go another way." There is Arthur, the menacing crime boss who jogs around his turf, tolerant of Angel because youth "has to see what is out there" and confident that Angel will mend his honest ways in the end. There is Moishe, the former labor organizer who has become a living statue on a street corner and lectures prostitutes never to give away their product for free. And there is the former reporter who complains that his novels are too readable to be published and his curse is that "nobody has to interpret me."

The dialogue snaps, crackles, and pops. Hentoff, who is a jazz critic and social-commentary columnist, is pitch-perfect on the speech patterns and inflections of street lingo. His prose is hard-edged with the staccato beat of the big city.

Take the opening paragraph:

"The detective's voice was low and flat. 'You have the right to remain silent. So long as you can stand the pain.'"

Later the detective, who is a good guy, looks at the arrogant, taunting suspect, and thinks "how satisfying it would be to step on him and feel the crunch."

There are 62 mini-chapters in The Man From Internal Affairs as Hentoff uses quick takes and scene shifts in jumping from character to character and subplot to subplot. The result is a choppy narrative.

In the end, the character, street scene, and dialogue cannot compensate completely for the thin plot. Almost but not quite.

MAGDALEN NABB, like Hentoff, uses the police procedural as a serviceable vehicle for her own special sensibilities. In her case, the standard police procedural, with its gritty realism, is transformed into a stylish mystery with delicacy and elegance. Her novels glow with the warmth and charm of their Florentine setting. Death in Autumn (Scribner's, $12.95) is the latest.

Her policeman is pudgy, shrewd Marshal Guarnaccia, of the Florence Carabinieri, who does not dismiss the report of two young Swedish tourists who say they spotted a woman's body in the Arno while unrolling their sleeping bags to spend the night on the Ponte Vecchio.

Divers find the body of a woman, naked except for a fur coat. She had been manually strangled, wrapped in the coat, and dumped in the river. Yet no woman has been reported missing, and more than a week is to pass before Guarnaccia stumbles on the identity of the corpse while making a routine round of hotels.

The dead woman is Hilde Vogel, a German in her late forties, who had lived at a respectable residential hotel in Florence for 15 years, dressed well, and took periodic trips. She was a loner whose only confidant seemed to have been the night porter, who heard of a former lover and rival for his affections in their early- morning talks.

Guarnaccia's investigation also raises some questions: Why did Hilde send monthly registered letters to Germany? Why did she keep the decaying country villa inherited from her father and now rented to young, drug-oriented foreign students? Why did she allow her son to be raised by her hated mother-in-law?

The answer to these questions leads Guarnaccia to a poignant tale of a woman used by an amoral man who believed himself to be a super-criminal. Without evidence for the courts, Guarnaccia cannot bring him to justice and can only wait for time to take its punishment. It does.

WHERE ELSE would you discover that the mother of Sarah Caudwell -- the author of those witty, sophisticated mystery novels that have won critical acclaim and reader loyalty -- was the woman on whom Christopher Isherwood modeled Sally Bowles? Or that Jack S. Scott, the author of the Inspector Rosher series, was once Silver Johnny Gray, the Singing Cowboy? Or that Tony Hillerman, author of the Edgar-winning series with Navajo Indian policemen, was raised among the Pottawatomie and Seminole Indians?

All these bits and pieces are gleaned from the updated second edition of Twentieth-Century Crime and Mystery Writers (St. Martin's Press, $69.95), edited by John M. Reilly. Its 1,094 pages are packed with information on some 640 writers (109 have been added while 78 were eliminated from the first edition).

This is a reference book that offers more than lists of titles and dates. For each author, there is a signed critical essay, a researched biography, and, when possible, comment from the writer himself.

Still, there are limits to the information in this compendium, broad as its scope may be. The second edition does not reveal the identity of the pseudonymous K.C. Constantine, who is the J.D. Salinger of mystery writers in his avoidance of interviewers or publicity. His seventh novel in the superb series (begun in 1972) featuring Mario Balzic as police chief of Rocksburg, Pa., soon will be released with the title Upon Some Midnights Clear.

Twentieth Century Crime and Mystery Writers notes that Constantine "jealously guards his anonymity" while "apparently living somewhere in the southwestern Pennsylvania area."

THE CORPSE half-lying on the bed in a room at the Black Swan Hotel stirs old memories for Detective-Inspector Luke Thanet in Dorothy Simpson's Last Seen Alive (Scribner's, $13.95).

Thanet recognizes the golden girl who was the most popular and beautiful in the class ahead of him in high school. What had happened in the last two decades and what had brought Alicia back to her hometown and violent death?

Alicia had left Sturrenden after the suicide of the young man who had been th most gifted in her small circle, which produced a world-famous violinist and a successful lawyer. Testimony at the coroner's inquest into the youth's suicide had stressed his despondency after Alicia decided to break off their relationship.

Thanet follows Alicia's life to London, marriage, widowhood, and a successful career as the owner of an employment agency. But he is to find the roots of her murder lead back to Sturrenden and the high school days of the past.

Simpson's mysteries are beautifully crafted and turn on character and psychological insight. Thanet is not a static character. Over five books, we have followed his personal problems from a bad back to his strong-willed wife's desire for a career. This time his son may have been involved in a glue-sniffing experimentation that takes the life of a school- mate.