BOOK REVIEW : Delaware Is Back on the Trail of Truth--and in Top Form : PRIVATE EYES <i> by Jonathan Kellerman</i> , Bantam, $21.50, 496 pages

Imagine that Philip Marlowe--a seeker after truth, worldly wise but noble, undaunted by even the darkest secrets of the heart or mind--has been reincarnated as a Beverly Hills psychologist in the ‘90s, and you will have a good take on Dr. Alexander Delaware, the shrink-turned-dick in Jonathan Kellerman’s latest psychological whodunit, “Private Eyes.”

It’s the sixth outing for Delaware, but there’s no flagging of energy or thinning of the blood here. After all, Kellerman’s beat is the human psyche, and he knows that he has tapped into an inexhaustible source of mystery, passion, intrigue and horror.

And that’s why, at nearly 500 pages, “Private Eyes” is a page-turner from beginning to end.

The patient in “Private Eyes” is a beguiling young woman named Melissa Dickinson, and the initial diagnosis is pavor nocturnus --night terrors.

We meet Melissa when she is 7, an enchanting but tormented little girl, and she reappears at 19 with a problem that gives more literal meanings to the Latin phrase--Melissa and her agoraphobic mother, a disfigured beauty named Gina, are caught up in a new set of terrors that turn out to be all too real.

Kellerman is adept at the conventions of detective fiction--he gives us a rich but secret-ridden family from San Labrador (a very lightly fictionalized version of San Marino), a faithful family retainer, a gold-digging second husband and a predatory villain whose black heart is filled with “acid-and-blood fantasies,” all of them riding a kind of diabolical merry-go-round of betrayal.

Delaware’s partner-in-crime-detection, Milo Sturgis, is a more conventional rendering of the Marlowe-esque private investigator: “Like someone better not say the wrong thing to me,” Milo warns, “or they’re gonna end up sucking the sidewalk.”

Together, Delaware and Sturgis, psychologist and detective, penetrate the most intimate secrets of the Dickinson family--and every secret is only the tip of an iceberg-sized mystery, which is appropriate in both the detective genre and the practice of clinical psychology.

All of the psychological trappings are perfectly authentic, of course--Kellerman is a psychologist in real life, and he wrote a couple of books on child psychology before turning to detective fiction.

As a result, Kellerman can’t resist the temptation to allow his protagonist to lecture us now and then--”Shrink’s reflex,” as Delaware says of himself.

“Play is fantasy,” Delaware explains. “The theater of childhood. . . . It’s a kind of self-hypnosis that’s necessary for healthy growth.”

Still, Kellerman mostly tells his tale in the approved narrative style of hard-boiled fiction: taut sentences, sometimes one or two words in length; dialogue that fairly hums with hidden meanings; a setting in the bleak moral landscape of Los Angeles and frequent bursts of the brand of metaphor and simile that is the trademark of the genre.

“A cello and a piccolo” is the shorthand depiction of a grown man and the young girl who is his charge, and “the marbled look of a tea-soaked egg” describes the flesh of a beautiful woman whose face was acid-scarred and then surgically corrected.

The plot kicks into gear when Gina, a recluse who favors the most exotic strains of travel literature, suddenly disappears.

Delaware and Sturgis embark on a quest to find her, all the while casting suspicious sideways glances at Gina’s sleazy husband and his personal trainer, a pair of conniving psychiatrists who are treating her for agoraphobia, and the sinister fellow who once sent a hireling to splash acid in her face and is now back on the streets.

At the end of the quest is a revelation that functions as the psychological equivalent of Grand Guignol.

Kellerman came up with a clever gimmick when he invented the character of Delaware--psychology is detection, after all--but it’s a gimmick that still works and still rings true.

“Strange is your business, isn’t it?” one of Kellerman’s characters asks Alex Delaware, and--as any psychologist will tell you, and as we learn in “Private Eyes”--the answer is: Yes, very strange indeed.

Next: Richard Eder reviews “Birdsong Ascending” by Sam Harrison (HBJ).