"You have to disguise yourself in order to unmask society," says Gu nter Wallraff. "You have to deceive and pretend to find out the truth."

Acting on that credo, West Germany's most controversial journalist has fashioned an unusual career by employing his impersonation skills to infiltrate murky sectors of society and writing bestselling books about his experiences. He has posed at various times as a messenger, a right-wing activist and an insurance salesman to expose illicit practices, and in the process has become celebrated as a heroic muckraker by his admirers and castigated as a shameless voyeur by his critics.

Wallraff's latest incarnation as an immigrant laborer in West Germany has produced a stunningly popular account about the poverty, prejudice and ruthless exploitation endured by many of the 1.5 million Turkish "guest workers" here. The book, titled "Ganz Unten," or "At the Very Bottom," has sold nearly a million copies within a month, becoming one of the most successful publications of the decade.

For more than two years, Wallraff lived as a stranger in his own country. He adopted the pseudonym "Ali" and disguised himself with a dark wig, mustache and thick accent. While hauling heavy stones at construction sites, cleaning toilets at a McDonald's hamburger outlet and sweeping coke dust in steel mills, he discovered what he called "a piece of apartheid" in West Germany.

As a Turk, Wallraff said, he found himself scorned as a "mule driver" and had a difficult time getting served in pubs and restaurants. When he asked an employer to compare German prejudice toward Turks and Jews, he said, he was told bluntly, "We won't gas you all. We need you to work."

In March 1983, Wallraff began his odyssey by appearing at an election night victory party in Bonn given by Chancellor Helmut Kohl's Christian Democrats. He quickly encountered his first taste of racism, he said, as a party official sarcastically remarked, "How international we are tonight! Even the Caucasus is celebrating with us."

Later, Wallraff -- alias Ali -- placed ads in newspapers soliciting employment as a "foreigner, strong, looking for work, no matter what kind, even heavy and dirty, even for little money."

His proposal, according to his book, took him across the country in a multitude of roles: He shoveled fish meal, he tried his luck as an organ grinder, he even signed on for a time as a guinea pig for drug experiments by a pharmaceuticals manufacturer. In each instance, he wrote, he was struck by the depth of hostility shown to foreigners by his fellow Germans.

Wallraff's most damning indictment focuses on what he described as the nefarious world of West German businessmen who assemble undocumented Turkish workers and hire them out to factories and construction sites. The companies are happy to acquire part-time manual laborers without having to pay union wages or social security costs. The so-called "slave traders" amass fortunes by collecting lump wage payments and doling out pittances to the workers, who are reluctant to complain because of their illicit status.

His book chronicles in detail his dealings with one of these human traders, whom he calls "Adler." As a prototype of a man willing to exploit "the poorest of the poor," Adler leases his unsuspecting Turkish workers to major steel and chemical companies to perform hazardous or exhausting tasks.

At a Thyssen steel plant, Wallraff wrote, he was instructed to clean a coke mill. The workers were given no protective masks, and soon Wallraff found "every breath is torture. You don't just inhale the coke dust, you swallow the stuff, and it chokes you."

With the help of actor friends, Wallraff concocted a highly dangerous job repairing a nuclear power plant. He secretly recorded the transaction in which Adler agreed to send eight men into a high-radiation zone where they could have suffered serious injury. The trader showed no hesitation in consigning the migrants to their dangerous duties once he was assured of a $40,000 fee, Wallraff said.

The enormous popularity of Wallraff's saga illustrating the grim underworld of migrant labor has been attributed by some commentators to public anxiety about persistent high unemployment and about the temptation to blame the Turks for taking jobs away from Germans, even the menial and dirty tasks that find few takers.

Wallraff's own provocative investigating style, marked by left-wing idealism and disdain for corporate abuses, has attracted a remarkable following ever since he wrote an expose' about the Springer publishing empire after bluffing his way into a job with the right-wing, mass-circulation daily Bild.

His name has become so closely associated with hoaxes and hoodwinking that it has evolved into a verb -- as in "to Wallraff" a person or an organization.

Wallraff's penchant for disguising himself and gleaning information through pretense also has prompted lawsuits from several of his victims.

But so far, he has been acquitted on all charges of forged identity papers and wrongful assumption of authority because West German judges have ruled that violating a law to uncover more serious crimes is permissible.