OVER THE EDGE By Jonathan Kellerman Atheneum. 373 pp. $17.95

OVER THE EDGE is the third outing for Alex Delaware, the crime-solving Los Angeles child psychologist of the very successful When The Bough Breaks and Blood Test. This time the case begins with a maniacal middle-of-the-night call from a young former patient, Jamey Cadmus. Within hours the teen-ager has been arrested at the scene of the mutilation murders of his mentor and lover, Dig Chancellor, and an anonymous street hustler, and held as the serial killer the headline writers have dubbed the Lavender Slasher.

The defendant is heir to a major Southern California construction business fortune, a genius-IQ misfit whose psychological deterioration has required his institutionalization. Since Jamey was found with the murder weapon in his hands, his attorney decides the job of the defense is simply to save him from the gas chamber. Delaware is hired to establish Jamey's diminished mental capacity.

Naturally, the case is not nearly as simple as it seems. There are anomalies in Jamey's mental condition that throw into doubt his capacity to commit the crime, let alone understand it. A friend of similarly doubtful mental stability -- they were enrolled together at a UCLA program for precocious children -- has absconded with evidence. Chancellor's bad-seed-ex-cop bodyguard is bent on revenge. Jamey's family closet has more skeletons than most families have shoes. Even his lawyer's driver has a few screws loose. And there is the conspiracy to develop environmentally contaminated land that motivates several of the principal players. Murder, drugs, greed, various styles of illicit but not very explicit sex -- the usual ingredients are here, given new vitality by the psychologist-narrator's unusual perspective.

Kellerman self-consciously emulates Raymond Chandler in his use of the landscape of the City of Angels, but his descriptions resemble writing-class exercises more than the sweltering, rotten Southern California dioramas Philip Marlowe inhabited. Bay City was a fully-imagined region of hell, not a collection of entries from last week's "What's Happening in Los Angeles" piece in the Sunday travel section. Although Delaware drives a Seville, his depictions of freeways, deserts and buildings evoke no image so strongly as that of Kellerman (in a BMW, not a Cadillac) researching "interesting" venues. In their Chinatown-style atmospherics and their evil-bloodline resolutions, Kellerman's books resemble not Chandler's but those of another Southern California crime writer, Ross Macdonald, whose Lew Archer stories were similarly talky and broody. And so many pages are devoted to learned discourse on the effects of mind-altering substances you think more often of the pharmacological excursions of Andrew Weil than of the adventures of the great L.A. detectives.

KELLERMAN'S yuppified investigator is a mixed blessing for his creator. Having been made a psychiatrist, Delaware must behave like one, which is to say he does have a tendency to run on. Though he is sufficiently alienated from his profession not to practice it full-time, Delaware is not disaffected enough to be a rebel, to have original or iconoclastic opinions that might make him a livelier character. In fact, happily married, well off, secure professionally, he has no problems whatsoever, no demons of his own to struggle with. The evil he confronts holds no fascination or risk; he is not driven by a passion for justice, an obsession with the terrors of the mind, a weakness for booze or women or money. In Over The Edge, at least, he is motivated by affection and professional responsibility more than the nearly accidental jeopardy of the earlier novels, but he is still basically outside the real margins of the story. On the other hand, a squeaky-clean, pedantic, monogamous, upwardly-mobile Boy Scout makes a nice change from the hairballs featured in most crime novels. I suspect it is his very identity as a DINK (double income, no kids) that makes him so attractive to so many readers.

Kellerman is one of those genre writers who aspires to be taken seriously as a stylist. His ambition is expressed in studiously grammatical prose dressed with occasional metaphorical and adjectival flair. Whether you like his writing or not will depend on your tolerance for such concoctions as these: " -- Pseudosenility -- . That recalled something -- the shadow of a memory -- but it darted through my mind like a minnow and hid behind a rock." Or, "I stood up, pressed my palms against the green walls of the interrogation room. The plaster felt soft, as if weakened by the absorption of too many lies." Or, "The blacktop rose before me, vacant and glistening, an arrowhead aimed infinitely at the horizon."

Kellerman enjoys a knack for creating complex plots and, with the important exceptions of Delaware, his shadowy musical instrument-building wife, and his best friend, an out-of-the-closet gay L.A.P.D. homicide investigator, believable characters. Jamey, especially, is a triumph, since truly mad characters are especially difficult to make sympathetic and compelling, while the jelly-spined uncle and saccharine aunt who are his guardians, the steely family attorney, their amoral retainers, and a shadowy pair of murderous bikers are so vividly drawn Kellerman approaches the boundary of the psychological horror story. :: John Gabree, a screen writer who lives in Los Angeles, often reviews mysteries and suspense fiction.