'I' IS FOR INNOCENT.

Sue Grafton. Henry Holt. 286 pages. $18.95. At the end of last year's " 'H' is for Homicide," Kinsey Mill-hone was fired from her part-time job as an investigator for California Fidelity Insurance, courtesy of an insufferable efficiency expert who took an instant dislike to her. "In the weeks since I'd been terminated," Kinsey recounts, "I'd gone through all the stages one suffers at the diagnosis of a soon-to-be-fatal disease: anger, denial, bargaining, drunkenness, foul language, head colds, rude hand gestures . . ."

Needing office space in a hurry for her private detective work, Kinsey rents a spare room in a law firm, serving subpoenas to make ends meet. But naturally, it doesn't take long for her to get involved in another big case, and Sue Grafton's " 'I' is for Innocent" is one of Kinsey's most electrifying adventures to date.

Six years ago, Kenneth Voigt's ex-wife, Isabelle, was murdered. Her estranged husband, David Barney, seemed the obvious suspect, but he was acquitted by a jury. Now David is busy spending the dead woman's considerable fortune, and Kenneth is concerned that there won't be any money left for the daughter he had with Isabelle. Now he has filed a civil suit of wrongful death against David.

Unfortunately, the private investigator Kenneth and his lawyer were using to gather proof against David suddenly drops dead. Kinsey is brought in to finish up, but as she interviews the parties involved and

uncovers new evidence, some serious reservations about David's guilt emerge. She then begins to doubt that Kenneth's ** first private eye died of natural causes. Was he killed because he got too close to finding the real solution to Isabelle's murder?

Ms. Grafton excels at making even the most minor characters memorable through nicely detailed descriptions (an R. N. is depicted as "the kind of nurse who assured you your tetanus shot would feel like a little bee bite when in truth it'd raise a knot on your arm the size of a doorknob"). And the finale, which features a showdown between Kinsey and Isabelle's killer in a pitch-black room, is a knockout. " 'I' Is for Innocent" is another irresistible installment of Ms. Grafton's series.

Ms. Trowbridge is a writer living in Baltimore.