THE ANGEL OF DARKNESS.<i> By Caleb Carr</i> .<i> Random House: 560 pp., $25.95</i>

When it comes to fence-sitting, American culture has performed a pretty shaky balancing act of late. What shall it be today: tactful or tabloid, elitist or trashy, restrained or over the top? (Princess Diana did die for such sins.) Meanwhile, silly mongrelizations and moronic oxymorons--”docutainment,” “creative nonfiction,” “fictionalized memoir” (increasingly the lazy person’s substitute for autobiographical fiction), even “network news”--swing their legs to one side, then the other, of the treacherous divide. And back.

Yet here’s Caleb Carr with “The Angel of Darkness,” attempting to accomplish again what he did in “The Alienist,” his last novel: Walk straight, upright and gracefully along the fence’s very edge. It’s hard to write historical fiction, his particular mixed forte, without insulting either part of the blend, and in the three years since his surprise bestseller, the ambivalence of the culture around him has intensified so much so fast, it’s amazing what he pulls off. His acrobatics betray hardly a wobble or misstep.

Although working one’s way through Carr’s hefty book is like reading history, he manages to make historical figures appear to spring full-blown (and full of odd humanity) from his imagination. In truth, real people, especially of the world-changing variety (why, just look at the Spencer family), usually turn out to be even more interesting than made-up ones. And Carr’s invented characters have, with few exceptions, both the surface and the internal cast of reality.

Still, “The Angel of Darkness” is also a story about storytelling. As “The Alienist” does, it opens in 1919, but about five months later, and flashes back to 1897, the same year in which Carr set the serial Beecham murders and introduced the hodgepodge group centered on Laszlo Kreizler, the pioneering doctor of the earlier book’s title, that solved them. Having barely recovered from the gruesome experiences and emotional wounds of that investigation, our heroes (and heroines: check one for 1997) are confronted with another violent criminal. In many aspects, this case is more horrifying than the first. And this time, rather than making the narrator the crotchety, hard-bitten journalist and alienated aristocrat John Schuyler Moore, a sensibility arguably not too distant from his own, Carr develops the persona of little Stevie (nicknamed “the Stevepipe”), the motherless street urchin salvaged from a life of dead-end crime and corruption by the good doctor in “The Alienist.”

Having reaped the benefits of life in the multicultural, egalitarian Kreizler household (check two), the adult Stevie has successfully gone into business as a tobacconist but is suffering from smoking-induced emphysema (check three) at the same time that he pens this tale. His voice, both young and older, allows Carr to concoct a style that is part Little Rascals and part 19th century dime-store novel, producing a rough-edged naivete and abashed sentimentality that stops just short of bathos.

Referencing the cartoon strips and popular serial fiction of the time, the book ends each chapter on a suspenseful note and plays a game of ominous foreshadowing and contemplative retrospect straight out of Gothic horror (the only irritant is Carr’s half-hearted effort to exhume the ungrammatical vernacular of a Stevie by his replacing “that’s” with “what’s” with tick-like frequency).

Because Carr’s interest remains an era teetering on the edge of modernity--the dawning of modern consciousness in particular--this is as much a psychological detective story as it is Conan Doyle. The question from the beginning is not whodunit but why, and when, where and how the agent of darkness will strike again. Revealing the villain early on leaves a writer with the challenge of sustaining a sense of mystery and dramatic tension. Through the observations, discoveries and confusions of his idiosyncratic detective squad--also back are the feminist Sara Howard, now proprietress of her own detective agency; Cyrus, the powerful, good-hearted Negro; and the cop brothers Isaacson--Carr deftly scrutinizes “the secret sins of American society” and the perpetual proposition that the greatest mystery is the human mind.

Especially, in this story, the mind of a female villain. It had to be a she--so dangerous and self-deluded she gives even Dr. Kreizler pause. Freudian theory and its offshoots originally developed out of case studies, but Carr takes the theoretical results and spins them back into individuals--otherwise known as characters. It’s brought him Libby Hatch. After the beautiful wife of an abusive Spanish diplomat goes to Sara seeking help in locating her kidnapped 18-month-old daughter, Ana, the trail quickly leads to Mrs. Hatch, a former child nurse with a string of youthful fatalities in her past. Kreizler & Co. fear that the baby is about to meet a similar end, a threat made all the more frightening by the fact that the ego of Libby, who inhabits a house she has equipped with a subterranean hideaway, is unconscious of the murderous instincts of her id.

Wily, seductive and disturbed, she sets her pursuers on a course that takes them not only all over New York City but also to the decadent “pleasure market” of Saratoga, its casinos and its race track and eventually to the factory-scarred upstate wilderness beyond. Although the courtroom drama that ensues features Clarence Darrow, the book also has cameos by Teddy Roosevelt, Elizabeth Cady Stanton, portrait artist Cecilia Beaux and other luminaries; what Carr is really doing is drawing on the present to illuminate the past, a reverse process from most history writing, whether imaginative or nonfiction. His archetype is therefore less Medea than Susan Smith, down to the “lunatic Negro itinerant” she accuses of having killed two of her own children and disabled another.

Carr’s past is neither better nor worse than our present. Its vicious downtrodden, too, are “darlings of the moneyed Bohemian crowd, who shared their craving for cocaine and liked to come down and slum it” (there are no perversions they haven’t seen) and draws on period feminist ideology to understand Libby Hatch. Instead, it is depressingly the same. Almost. Carr’s fin de siecle America operates according to a moral code and sense of honor, which seems sweetly quaint compared to the “Natural Born Killers” ethos of today.

One of the novel’s pleasures is that it lays out a historical map of a New York that should be familiar to anyone who knows the city today, albeit faded, changed or in disguise: Grace Church and the Fulton Fish Market, Pete’s Tavern and the Old Town Bar, hospitals and erstwhile mansions, Fifth Avenue and Broadway, live here as in the canvases of the urbane American Impressionists and Ashcan School painters Carr seems to evoke. It renders Carr a tour guide of the idyllic greenswards, satanic waterfronts, rooftop jungles and domestic havens that continue to function as cities’ epitomes of heaven and hell, of the poles of human nature at its happiest and most distressed.

Geography is a whole century’s worth of destiny in his capable hands. A nocturnal visit to the Brooklyn Navy Yard, where battleships are secretly being constructed in anticipation of war with Spain, is reminiscent of descriptions of Los Alamos, and we learn that in Cuba the Spanish are herding rebels into places called “concentration camps.” While advanced medical men like Kreizler are laying the groundwork for psychoanalytic knowledge and personal insight as we know it, the 20th century’s other distinctive achievement, the advancement of technology for warfare and global destruction, progresses along a parallel track.

In a complex narrative like this, though, architecture can’t help but serve as a metaphor, as late-19th century observers increasingly perceived it, for female entrapment, for labyrinthine emotions, for the monumental differences between rich and poor. In Libby Hatch’s architectonic madness, Charlotte Perkins Gilman’s 1899 feminist classic “The Yellow Wallpaper” comes to mind, and the sadder stories of Edith Wharton, forever building mazes of feminine imprisonment and masculine misunderstanding. It takes a visit to a Vanderbilt’s mansion to illuminate why the likes of well-born John Moore and Sara Howard offend contemporaries with their classlessness.

We were given a lot of insight into Moore’s embittered populism, along with his life story, in “The Alienist,” and “The Angel of Darkness,” of course, is Stevie’s book. It seems clear Carr’s next installment will be Sara Howard’s narrative and autobiographical turn: Carr hints just enough at childhood traumas, romantic misadventures and complicated family history on her part in this novel to whet the appetite for a sequel. Setting the stage with allusions to her past, “The Angel of Darkness” steers Carr toward a promising future.