When the Dick Is a Dame : ‘I’ IS FOR INNOCENT <i> By Sue Grafton</i> , <i> (Henry Holt: $18.95, 286 pp.) </i>

When Sue Grafton made her debut as a mystery novelist in 1982 with “ ‘A’ Is For Alibi,” her sleuth-narrator introduced herself as follows: “My name is Kinsey Millhone. I’m a private investigator, licensed by the state of California. I’m thirty-two years old, twice divorced, no kids. The day before yesterday I killed someone and the fact weighs heavily on my mind.”

In her newest satisfying search for truth and justice, “ ‘I’ Is For Innocent,” Kinsey is as candid, observant, funny, loyal and determined as ever. But the intervening years have wrought some changes--in her universe and in the real world, where the mystery field has become so overrun with women private eyes one can only marvel at the ingenuity of the authors and the tolerance of readers that keep them all gainfully employed.

Grafton was not the first woman writer to send a female into the former boys’ club of private eyedom. In the early 1970s, P. D. James presented London’s Cordelia Gray in “An Unsuitable Job for a Woman,” and later that same decade, Marcia Muller gave us San Francisco sleuth Sharon McCone in “Edwin of the Iron Shoes.” But Grafton did create a character who, over the course of eight novels, has developed into one of the most popular of fiction’s gumshoes.

Developed is the key word. Though she hasn’t aged very much--she’s still in her early 30s--she’s less brash, less rash and, following the lead of both Chandler’s Philip Marlowe and Ross Macdonald’s Lew Archer, she has slowed down what was once the fastest mouth in the West.

Grafton’s earlier books used the same pattern as did those of Chandler and Macdonald, with the detective solving the crime by moving from one character to another until all the clues fall into place. Lately, however, she’s made an apparently conscious effort to vary Millhone’s equations.

Though her novels continue to be who-done-its, involving domestic violence, Kinsey’s last job, “ ‘H’ Is For Homicide,” in which she went undercover to get information on a murderous Chicano hoodlum, contained more action and adventure than detection. Prior to that, “ ‘G’ Is For Gumshoe” featured a mysterious hit man who placed the detective in jeopardy for nearly all of the novel, with self-preservation shoving the sleuthing into second position.

“Innocent” marks the detective’s return to a more conventional PI case, which should be satisfying to the purists. Fortunately, there are enough unconventional trappings and diversions to satisfy everyone else.

Four alphabet letters back, in “ ‘F’ Is For Fugitive,” Kinsey was hired to prove the innocence of a young man convicted of murder. Here, she’s asked to prove the guilt of a man declared innocent. David Barney has been tried and acquitted of the shooting death of his architect wife. But a number of people, including Kinsey’s client, the victim’s former husband, think justice’s blindfold was on a bit too tight. The ex-spouse has brought a wrongful-death civil suit against Barney that, if successful, would not only discredit him but take away the fortune he inherited as a result of his wife’s death.

It’s Kinsey’s job to provide her client’s lawyer with supporting evidence, but when Barney offers her seemingly irrefutable proof of his innocence, she’s stuck with the disturbing notion that somebody else murdered the woman. The list of suspects includes the victim’s bitter twin sister, her passive (and aggressive?) former partner, his tart-tongued wife and, of course, Kinsey’s client.

From time to time, Grafton interrupts the presentation of this clever, complex puzzle with a secondary plot in which Kinsey’s gentlemanly, octogenarian landlord, Henry Pitts, is being driven crazy by an open-ended visit from his ultra-fastidious older brother. This humorous diversion, though obviously less lethal and compelling than the main plot, sets Kinsey apart from the private detectives of Hammett and Chandler, and thereby provides a clue to her popularity. Those earlier detectives weren’t just male, they were invincible males, impervious to booze and beatings and unconcerned with the daily affairs of their fellow human beings. And they were loath to discuss their private lives in print.

Kinsey’s personal history intertwines with her narratives. Friends die; romances begin and end; her prized VW Beetle is pulverized. In “Innocent,” she’s suffering the aftermath of the abrupt termination of a longtime business association with an insurance company. These are things we can all relate to. In refusing to limit her heroine to just the case at hand, Grafton has moved the private eye story closer to real life than did either Hammett or Chandler, for all their talk about realism.

As has been noted, the author and her irrepressible heroine have made it possible for women writers to explore (and possibly take over) the hard-boiled detective arena. More important, she has breathed new life into the nearly moribund sub-genre, allowing fictional creations of every gender, nationality, sexual preference or political agenda to continue to sleuth down mean streets in the pursuit of crime.