“Who is this Essad Bey?” Trotsky asked in a 1932 letter to his son. By then, this mysterious writer had written bestselling biographies of Mohammed and Stalin, a book on the oil industry in Baku (in the early 20th century the Texas of the Caucasus), and a steady stream of articles on literary and political subjects from Tolstoy and Dreiser to the Ottomans and Americans (“American History in Five Hundred Words”).

In one photograph he appears as a sporty figure in a fez; in another he is dressed as mountain warrior with a dagger at his waist. He claimed descent from Muslim princes, but others alleged he was the son of an oil millionaire in Baku, a nationalist poet, or a Viennese writer who died in Italy after stabbing himself in the foot.

Not many biographers have to begin their projects by first figuring out the identity of their subjects. But in order to write “The Orientalist” (Random House, 464 pages, $24.95), Tom Reiss traveled to 10 countries in search of Essad Bey, aka the bestselling novelist Kurban Said, author of “Ali and Nino,” a 20th-century literary classic.

Mr. Reiss’s book chronicles the adventures of a biographer, disclosing the process by which he discovered that in fact his subject was Lev Nussimbaum, a Jew born in Baku in 1905, an escapee via camel caravan from his native land, which Stalin (once a guest in Lev’s own home) was plundering and devastating. Lev would die of a rare blood disease in 1942 in Italy, two weeks too late to take advantage of doing the radio broadcasts that Ezra Pound had arranged for him.

Lev (as his biographer calls him) yearned for the pre-World War I world. Like Disraeli and a generation of 19th-century Jews, Lev was an orientalist – a mystic, really, who believed in a kind of pan-Semitic peopling of the East. Although Lev assumed the identity of a Muslim – even converting to the religion – and married an American wife without telling her that he was born a Jew, he was something other than an imposter. Among friends, he would even joke about his assumed identity, and anyone who became Lev’s friend quickly realized that his father, who lived with Lev, was hardly the Muslim prince Lev claimed as his progenitor.

Lev is best understood as a writer. All else – his marriage, love affairs, politics – was at the service of his imagination. Life for him was something that had to be brought to book. Stalin, in Lev’s biography, was not only the monster-totalitarian who destroyed the diverse world of the East and tyrannized his own people, he was also a gangster/bank robber and a friend of his mother, herself a revolutionary who committed suicide after marrying Lev’s oil millionaire father.

Or so Lev claimed. Mr. Reiss can sort out the fact from the fancies only up to a point. As he asks when he quotes Trotsky’s query: “Was it even clear that Lev knew the answer by this point?”

Lev was a bestselling author in Nazi Germany until Goebbels & Co. discovered his Jewish identity. After 1935, Lev could have stayed in the United States, even though his marriage had broken up, since he would have had no trouble earning his living. He was a prolific author who had already been translated into 17 languages.

But Lev was a monarchist. He had no more faith in the United States than he had in Weimar Germany. Democracy, to him, represented merely a cacophony of political factions. Kings had ruled the world for centuries, and so they should again. Dictators ran a poor second to kings, since they did not, in Lev’s view, hold power in trust for the people but only for themselves.

In fascist Europe, where Lev returned to live, he sought protection from those in power. So as late as 1938 he aspired to be Mussolini’s authorized biographer. At least Mussolini had shown some respect for the Italian monarchy.

Lev was no Nazi, but like Disraeli he might be called a racialist (Mr. Reiss shows how Disraeli’s novels dramatize a sense of Semitic supremacy that made the imaginative world of Essad Bey conceivable). Lev thought of himself as a “Man from the East, a realm of lost glory and mystery. He began to fantasize about a pan-Islamic spirit that would preserve everything from revolutionary upheaval.”

Lev carried with him what Mr. Reiss calls a “portable Orient,” which Lev would embody for the entertainment of his audiences. He was Zeliglike (Mr. Reiss alludes to Woody Allen’s movie) in so far as he seemed to be able to change identities without any sense of inner conflict. In Positano, Lev’s final destination, he enjoyed the admiration of a community that did not doubt his identity, finally erecting a gravestone that read “Mohammed Essad Bey.”

Mr. Reiss does not provide a scrap of evidence to show that Lev turned Turk because he repudiated his Jewishness. “Figures as diverse as Disraeli and the philosopher Martin Buber played a part in this relocation of the Jewish spirit to the realm of pan-Asia,” writes Mr. Reiss.

This is a lost world of the imagination that the biographer recreates with extraordinary aplomb. It appears in all its strangeness and wonder in the midst of the biographer’s own tales about his strenuous efforts to find out who Essad Bey and Kurban Said really were.

As a biographer, I especially enjoyed Mr. Reiss’s accounts of his efforts to entertain his interviewees. In one case, he had to visit a castle inhabited by a source who was writing lyrics for a musical. The trouble was she had never seen such a production. Had Mr. Reiss seen one? Not in a long time, he replied, but the obliging biographer then performed versions of “Singin’ in the Rain” and other classics of the musical stage – all while making his way to a freezing room stuffed with prized documents he could only peruse under natural light. (Ah that’s the trouble with those castle assignments, an arduous part of the biographer’s task).

For sheer reading pleasure, for insights into the biographer’s world, and for the rediscovery of a major literary figure (please, someone, reprint Lev’s biography of Stalin!), this book cannot be bettered.