THE BRITISH tend to know a good thing in literature when they see it. They got Half Moon Street first and made it a best seller. They also discovered Paul Therous first.

We Americans may be realizing at last that Theroux, who writes fiction (The Mosquito Coast), nonfiction (The Great Railway Bazaar), and occasional criticism (V. S. Naipaul) as if born to the word, is among the best American writers. Half Moon Street is his 22nd book and it shows some of the reasons why. It's nearly everything fiction should be -- diversion, psychological revelation, and social news. Furthermore, it is written in radiant language.

Half Moon Street consists of two short novels -- "Doctor Slaughter" and "Doctor DeMarr" -- about identity, power, and the fatal dangers of the world we live in.

Stories about lonely people who want more and find troule, they see strangeness and evil drifting through English-speaking society like a change of weather. Sinister processes are at work in the service of personal separation and cultural discord. Decay lies beneath the surfaces of social ritual and institutional facade. Humane qualities like compassion and affection are smothered by urgent drives for control and sensation. Not everything in Paul Theroux's tales is black. But nearly everything is seen as if at 2 in the morning. The first novel deals with a young American scholar in England who falls into a ring of international blackmailers and assassins while believing she is merely doing for money what comes naturally.

Dr. Lauren Slaughter (her passport says she is Mopsy Fairlight of Culpeper, Virginia) is a fellow at the prestigious Hemisphere Institute of London and so successful at dominating men through sex that she even pays her plumber that way.One night at a party in a conversation with a mysterious banker named Van Arkady she advertises her self-proclaimed nymphomania, and her life changes. Goodbye one-room Brixton flat, hall telephone, frozen toilet, East Indian neighbors.

While continuing her research into Persian Gulf politics and the path of petrodollars by day, Lauren moonlights at $250 a night in the employ of the Jasmine Escort Agency. Her clients, many of whom are Arab oil potentates, prefer sodomy or voyeurism. One repeatedly spends hours photographing Lauren riding an exercise bicycle. He also likes her so much he arranges dates with friends and gives her the fancy apartment she has always wanted. It's on Half Moon Street, across from London's Naval and Military Club. Thus Theroux's title.

It's tiring but exciting work. Because she jogs every day, doesn't drink or smoke, and eats only raw vegetables, Lauren can stay up half the night performing sexual acrobatics and still lead a discussion at the institute. Her master's in economics and Ph.D. in international relations make her the conversation equal of her patrons. Sex makes her better. The men never kiss her. They're fussy about their hair. Their needs are quick and odd. All they have is money and loneliness. Lauren disdains most of them. Superior always, she leaves them weakened, drained, animals made feeble and sleepy by their lust.

Everything seems to go perfectly.Lauren has $3,000 cash stuffed in an unused coffee pot, standing invitations to Kuwait, Bahrain and Qatar, and a regular date every Wednesday with Lord Bulbeck, a Labor Life Peer who knew Ben-Gurion and rides in a Mercedes flying a United Nation's flag. Lauren knows that she is someone extraordinary waiting only for recognition.

But Theroux's vision in these two novels is of conspiracies in which the ultimate victims are those who while manipulating others are manipulated themselves and drawn into violence.

Lauren has been a sexual shill for terrorism. She narrowly escapes being murdered. She is an accessory to the murder of the only man who saw her as a person rather than an object. With a dazzling close Theroux pulls together all his casual story lines into a knot of logic.

The assumptions and conceits of "Doctor DeMarr" -- double identity, criminal intelligence, social decay, the guilty as victim -- are almost exactly those of "Doctor Slaughter."

Gerald DeMarr, a middle-aged failed economist in a Boston Suburb, hasn't seen his twin brother George in decades. Suddenly George turns up one day, sick and desperate, pressing his face against the screen door and asks Gerald for a place to stay.

Reluctantly, Gerald agrees and leaves for a vacation at Cape Cod. He was once inseparable from George. Too many personal failures, too much festering envy, too many years of separation between young manhood and middle age have turned his feelings into indifference or hostility toward his lost brother. Gerald tells George to be gone from the house when he returns.

Instead, George is dead. Gerald finds him sitting upstairs in a chair, his arm full of punctures, drug paraphernalia nearby. Gerald buries the body in a landfill to avoid trouble and decides to trace his brother's recent past by means of a laundry ticket. He discovers that his twin was a successful impostor of a medical doctor who sold prescriptions to addicts and was on the verge of detection.

Gerald assumes George's identity. Despite its problems it's a better life than his own. He enjoys the dignity of being a "physician." The patients respond. He may even be better at it than George was. And he makes gestures toward correcting his brother's failures. But he slips.

"Doctor Slaughter" seems by far the better of the two novels. Gerald Demarr's motivation is almost mythic. Lauren's is concrete. "Doctor DeMarr" is just barely credible. "Doctor Slaughter" is as real as an escort service classified advertisement in the International Herald Tribune.

But each is successful. In Half Moon Street Paul Theroux combines unlikely elements -- mystery, character study, the Gothic nightmares of industrialized society -- in fiction that shimmers with implications, possibilities, and warnings.