IT's Ghostbusters with hard-boiled 'tec style grafted to science fiction in a wildly lunatic, bizarre adventure from the pen -- both words and illustrations -- of Gahan Wilson, whose idiosyncratic cartoons appear regularly in The New Yorker.

That's Eddy Deco's Last Caper (Times Books, $14.95). This is one of a kind. And that is what it probably should remain. Wilson's book is an original, a curiosity in the mystery fiction field, and there lies its fatal attraction. It cannot repeat itself. It takes a special taste to relish Wilson's special brand of humor. Let's just say that Eddy Deco's Last Caper is not for everyone.

Wilson begins with a parody of the hard-boiled detective school of the '30s and '40s -- a tough, wisecracking private eye with a bottle of rye stashed in the desk drawer, a bleached blond secretary, a shabby office in a rundown building with a creaky, perverse elevator and marble corridors "broken and badly fixed too many times like an old boxer's face," and a former girlfriend who says of her present racketeer boyfriend: "He is a rat, Eddy, but he's my rat."

The hard-boiled parody is a launch pad for the take-off of Wilson's antic imagination. It would be unfair -- and well-nigh impossible -- to give a summary of the zany goings on in Eddy Deco's Last Caper. Suffice it to say that furniture, toasters and lamps attack, the elevator pursues Eddy, buildings turn into spaceships, and the villains are aliens who belong to a tribe called the Badgize (pronounce it). As we said, it's wild.

More than 100 of Wilson's original illustrations are integrated with the text and are the brightest part of the narration (the prose parody often stoops to silly excess). The drawings are more than decoration. They function, quite effectively, to advance the story. It takes a little time to pick up on this device. On an early page, Deco is pursuing a bum who took a potshot at his client. There's a drawing with a blackjack in a big fist, and then the text picks up with a groggy Deco on the ground looking up at the big, flat feet of a policeman.

Eddy Deco's Last Caper is an experience that can be missed except by those who have a taste for extravagantly grotesque antics. They will find it weirdly wonderful.

Legal Tender

THE INDIANA judge who writes lawyer mysteries, Joe L. Hensley, has come up with a small delight to launch a new series. Fort's Law (Doubleday Crime Club, $12.95) is a tidy, compact tale told in the crisp and straightforward manner of a good legal brief.

Jack Fort, the narrator, is a most likable fellow. He came home from Vietnam with a slight limp and has left a high-powered Chicago law firm to try small-town practice in Scannelsville, Ind., a county seat with a population of 9,000. There Fort runs into an overbearing, opportunistic circuit prosecutor who boasts of running the county under "Windham's law." The newcomer from Chicago is to change that.

In Fort's Law, the lawyer's client is Jesse DeAlter, accused of murdering his rich old aunt for the family coal-mining fortune. Fort takes the case although he doesn't care much for his smug, arrogant client, who has been a doping, drinking, philandering wastrel. But then the scheming prosecutor, with his eye on a judgeship, may be even worse.

Judge Hensley, whose earlier series featured Don Robak, a crusading criminal lawyer, does not use court-room pyrotechnics a` la Erle Stanley Gardner. But his lawyers know how to turn a legal maneuver (Fort uses an oddball Indiana law on bail as a discovery tool to get an up-front view of the state's evidence).

The courthouse crowd offers some nice character cameos, including Fort's elderly law partner, who savors being part of a murder investigation as a break from routine legal practice. Fort's Law is unpretentious but impressive in its own low-keyed way.

School Days

JACK DWYER had not wanted to go to the 25th reunion of his high school class. And he certainly wishes that he hadn't when Karen, the girl he had dated in school and still the beauty of her class, collapses in his arms while they are dancing. She is not drunk but dead.

In The Autumn Dead (St. Martin's, $14.95), Dwyer must make a journey to the past to uncover the secrets that led to Karen's death a quarter-century later. This is the latest -- and one of the best -- in Edward Gorman's series featuring an ex-policeman who works as a rent-a-cop for a security firm while pursuing an acting career. Most of the time Dwyer has to be content with making a sales pitch on television commercials.

As the narrator, Dwyer has an appealing self-deprecating sense of humor. Of course, a 25th high school reunion leads to some musing on the passage of time and the loss of innocence and changes in the old neighborhood. In the remembrance of things past for Dwyer, there are some nice touches -- the clean smell of clothes hung on the line to dry, the fun of going downtown before shopping malls.

In The Autumn Dead, Dwyer and Donna, his love, seem to have settled into a comfortable romance after some rocky days. It's a nice relationship with affectionate banter between the two. The dialogue is snappy, and Gorman writes brightly and crisply with a jolt of surprise at the end.

On the Piazza

MAGDALEN NABB's Florentine series, featuring Marshal Guarnaccia of the Italian Carabinieri, has offered distinctive mystery novels with an elegant, literate style. The lovely Italian city of Florence, so beautifully captured in mood as well as scene, often has overshadowed the plot and the police work.

This time, in The Marshal and the Murderer (Scribner's, $14.95), Guarnaccia, the bulky, middle-aged Italian officer who bears resemblance to Georges Simenon's French Inspector Maigret, goes to a pottery center outside Florence to investigate the disappearance of a Swiss student. Her body later is found under a heap of potsherds at a pottery factory.

If the Swiss girl was reported missing in Guarnaccia's jurisdiction, her body has been found in that of another Carabinieri officer, big, outgoing, energetic Niccolini. It's a touchy situation, but the two policemen warily cooperate.

It is Niccolini, more aware of the ways of the world, who picks up the lesbian relationship between the dead woman and the companion who reported her disappearance to the Florence police. Was she murdered in a fit of rage by one of the men with whom she flirted outrageously?

But it is the slow-moving, meditative Guarnaccia who is to solve the murder by sorting out the town's secrets that go back to World War II, the partisans and German SS officers. Guarnaccia, who grew up in a small Sicilian town, knows how to mine local gossip and isn't surprised when anonymous letters arrive at police headquarters.

Again, as in the earlier novels in the series, Nabb's strength is the background and atmosphere, with a fascinating look at pottery making. It's just as well that the atmosphere is so evocative, because the plot is byzantine and convoluted beyond belief.

Jean M. White reviews mysteries for Book World on the third Sunday of the month.