A MORE UNLIKELY hero would be hard to find. Detective Inspector Alfred Stanley Rosher, a stumpy figure of a man with an ape-like body and brow to match, can be coarse, uncouth, rude, and churlish. His clothes fit the man: shabby blue serge suit, boxy-toed shoes, and scruffy bowler hat. When he pulls out a grayish, rumpled handkerchief to blow his nose, colleagues have been known to warn: "Take cover."

Yet Rosher has become, warts and all (perhaps because of those warts), a peculiarly endearing hero in the mystery-suspense series from Jack S. Scott. A Death in Irish Town (St. Martin's/Joan Kahn, $12.95) is the latest and arguably the best in this series, which began with The Poor Old Lady's Dead in 1976.

Scott blends black comedy with the police procedural, two elements that would seem as antithetical as oil and water. And he does it brilliantly, never missing a beat of suspense or a chance to shaft the hypocrisies, pretentions, and absurdities of life.

Over the nine books, Rosher has emerged from his blue serge and coarse manners to take on the flesh of a complex human being, fallible but likeable. No one is going to confuse this unpolished, rough-cut provincial British copper with an urbane, gentlemanly Scotland Yard man. But Rosher is shrewd and does his job well in his own way. Tough as he is, he can feel for other people and for himself.

In A Death in Irish Town, Rosher, who knows retirement is not too far away, returns to Irish Town, where he patrolled the streets as a young constable and was bedeviled by the rowdy clan of O'Haras.

Now a charred body has been found in a warehouse fire, and the victim turns out to be a grandson of Nellie O'Hara, the feisty Irish housewife of the early days. Nellie now is a cackling crone cadging drinks in the Ring o' Roses pub. Her niece, Mavourneen, then the lass with blue eyes, is the barmaid, and when her "bolster bosom" brushes Rosher as she passes, there is a stirring of old desire.

As Rosher wanders around his old beat, quite a few people get nervous and jumpy. Why are the truck driver and warehouse clerk upset when they can't unload their truck unobserved? What is the link between the homosexual couple and the bartender and his wife? Why all the fuss about the boat rented to tourists? Scott hooks the reader with questions that must be answered.

Rosher, crusty as he may be, can have his touching moments. At breakfast, Scott writes: "His portable radio stood where he always put it, on the kitchen table beside the ketchup bottle, tuned to a merry chat show which never claimed his ear -- it pushed back the silence, that's all -- except when a more urbane tone took over from the hysterically happy presenter to give the news bulletin." Fletch Flashback

FLETCH, the irrepressible, irreverent young man whose fun-and-games adventures have enlivened the mystery field over the last decade, returns in a back- to-the-future book about his start in the newspaper business.

This latest adventure, Fletch Won (Warner, $14.95), is the first chronologically. It covers the days of Fletch's callow youth as a cub reporter and copy desk editor. He barely hangs on to his job after he writes headlines such as "Western Can Co. Sits on Its Assets" and begins a routine wedding announcement: "Sarah and Roland Jameson, first cousins, are to be married Wednesday in a ceremony restricted to the family." Fletch argues that he has checked and found that neither the bride nor groom are adopted and that he is only following good reportorial techniques.

As a last-chance transfer, Fletch is sent to the society department, now into lifestyles and long features, and told to do a public relations story about a wealthy criminal lawyer who has announced a $5 million donation to the local art museum.

Then comes Fletch's chance for the big story. The lawyer is found shot to death on the News-Tribune's parking lot. Of course, Fletch will have to ace Biff Wilson, the obnoxious crime reporter, who has ties with the police department. But Fletch is to win, solving the murder and unearthing a big scandal to boot.

Gregory Mcdonald, who has won two Edgars for his Fletch series, is a consummate farceur. No one writes sharper, wittier repartee. His characters have an off-the-wall zaniness and yet can touch your sympathy. He is the master of orchestrating riotous comic scenes. In Fletch Won, our zealous reporter ends up naked in a swimming pool while a dotty woman leaves to take his clothes to the laundry. (He thought she would run them through the washer-dryer). This comes after he has been marinated in bourbon when mistaken for a thief at a liquor store where he had stopped to ask directions.

The latest Fletch outing offers only a thin plot. And at times the bouncy banter turns smart-alecky and worse. But Fletch is still a charmer, and Mcdonald has great fun with legal ethics and the newspaper business, which he knows well from his stint as a reporter on the Boston Globe. Cambridge Killer

LIZ CONNORS, a free-lance writer who turns free-lance detective, is going to win a lot of fans as the witty narrator of The Gemini Man (Walker,

Susan Kelly's maiden mystery is fresh and free-spirited with lively characters, snappy dialogue, on-location scenes from the Harvard Psychology Department to Cambridge single bars, and a suspenseful plot neatly handled if not that original.

Liz is a 33-year-old ex-English professor who is trying to earn a living as a free-lance writer. Her lover is a police homicide detective. So she knows whom to call when she discovers the battered body of the young woman who lived in an adjoining apartment. All Liz knew about Joan Stanley, the victim, was that she seemed a hard-working, quiet graduate student in psychology with a serious boyfriend on campus.

Liz is to learn that we really can't know our neighbors from casual exchanges as she starts digging into Joan Stanley's life for an in-depth story for a local weekly. This leads to a collaborative investigation with Police Lt. Jack Lingemann, her lover, who is handling the case. Other women are murdered before the explosive scene in which a killer with a twisted psyche reveals himself to Liz.

The police lieutenant and the free-lance writer make a credible and appealing pair of sleuths. Their romance has warmth without frantic grappling, with both relaxed and comfortable in the relationship.

Liz is a breezy narrator with a neat turn of phrase and wit. A singles' bar reminds her of "a cross between a Hieronymus Bosch painting and a scene from Annie Hall." Of the elderly widow and drug-user on the floor above: "The widow lived on Social Security and her husband's Cambridge Fire Department pension. The airhead lived on Quaaludes."

Kelly, an associate in management communications at the Harvard Business School, teaches communications at the Cambridge police academy and writes with an insider's knowledge of police procedure and forensic medicine. Sleuthing in Spain

BOTH RODERIC JEFFRIES and David Serafin have been writing series featuring Spanish police detectives. Inspector Enrique Alvarez works under the sunny skies of Mallorca for Jeffries while Serafin's Superintendent Luis Bernal is a big-city policeman in Madrid. They share little except their Spanish blood.

In Layers of Deceit (St. Martin's, $13.95), the easy-going Alvarez once more must deal with a death among the British expatriate colony, whose mores are shrewdly chronicled by Jeffries, who lives on the island himself. The characters need to be sorted out at first, but patience is rewarded as Alvarez scents murder to his superior's chagrin. The Spanish authorities would prefer not to be involved in affairs of the English colony.

Alvarez may like his brandies and siestas, but he is a dogged, canny investigator. Sometimes he must tinker with the law to achieve an ironic justice.

The Body in Cadiz Bay (St. Martin's, $13.95) is less successful than its predecessors in the Serafin series, including the prize-winning Saturday of Glory and Madrid Underground.

Bernal, the portly inspector with a mistress (now pregnant) and a devout wife who refuses to divorce him, gets entangled in some outlandish goings-on that include Moroccan terrorists, right-wing military officers and priests, a laser-beam gun as a murder weapon, and an underground spring that brings barren women to a Catholic convent to be blessed.

Eugenia, Bernal's wife, is at a convent near Cadiz Bay to pray for her husband's soul, when fishermen net the body of a man dressed in frogman's gear. He may have been spying on the joint United States-Spanish naval base. Meanwhile, back at the convent, priests and military officers speak in confidential tones and a frogman's suit is found by the underground spring. Things only get more complicated and silly.

Superintendent Bernal should return to Madrid and sanity.