Double jeopardy — it’s prohibited by law. Which means that once somebody has been found not guilty of a crime, he can’t be tried again by the state, even if damning new evidence turns up. But in recent years, acquitted homicide suspects have been getting sued in civil court — by family members demanding money damages to compensate for the ”wrongful death” of a murder or manslaughter victim. Furthermore, it takes a lot less evidence to nail somebody in a civil trial than in criminal court: none of that ”beyond a reasonable doubt” stuff.

A curious legal situation. And a nifty premise for Sue Grafton’s ninth mystery starring ”kick-ass private eye” Kinsey Millhone of Santa Teresa, Calif. — who’s hired to help build a wrongful-death suit against David Barney, acquitted two years ago in the nasty bullet-in-the-eye killing of his rich, estranged wife, Isabelle, celebrity architect and ”flaming narcissist.” If Barney loses, the millions he inherited from Isabelle will belong instead to Shelby Voigt, her 10-year-old daughter from a previous marriage. The chances of that happening? Surprisingly good, it seems — especially since a new witness has come forward: Barney’s prison cellmate during the trial, who’s now ready to swear that the slick, arrogant defendant admitted killing Isabelle. Once Kinsey goes into action, however, the case of Voigt v. Barney pretty much comes apart in her hands. Without intending to, she unmasks the cellmate witness as a flake — and probably an out-and-out liar. Her attempts to crack Barney’s iffy alibi have exactly the opposite effect. Worst of all, she begins to suspect that her predecessor on the job, an overweight shamus who dropped dead of a heart attack, was poisoned to prevent him from finding out too much. So it’s not long before Kinsey has to start wondering about the other folks — and there are plenty of them — with juicy motives for murder. The crippled sister who blamed Isabelle for her injuries, for instance. Or the over-the- hill architect who gave ungrateful Isabelle her first break. Or the ex- husband who’s still obsessed (to his new wife’s mortification) with beautiful ”Iz.”

Grafton’s ever-expanding fan club may find this nicely devious, firmly paced whodunit a little tame, a little old-fashioned after the wild rides of ”G” Is for Gumshoe and ”H” Is for Homicide. Aside from a duel-to-the-death showdown with the killer, there’s not much personal peril for Kinsey here. But in the course of her investigation she manages, as always, to fall into step with a steady parade of singular, astutely observed characters — from a recovering teenage alcoholic to a woman who designs headwear for cancer patients. And Kinsey herself, a lot more cheerful than that other 40ish feminist private eye out in Chicago, continues to be wonderfully agreeable company.

The best Kinsey Millhone yet? Not by a long shot. But, like Dick Francis, Robert B. Parker, and others who’ve scored their biggest hits with lesser work, Grafton may see this not-quite-amazing adventure become — thanks to momentum, hype, and a 225,000-copy first printing — the season’s best-selling mystery. B+