A shot-in-the-dark letter from Michael Korda to the Kremlin last year led to Simon & Schuster's forth-coming biography of Leonid Brezhnev.

Now Korda, who has also published books by Charles de Gaulle and Jimmy Carter, is in the unusual position of not knowing exactly who wrote the manuscript, though he knows the Russian author meets deadlines so well he makes most American writers look laggardly.

"I pretty much forgot about the letter to Brezhnev after I had sent it," says Korda, Simon & Schuster's editor-in-chief at age 44.

That first missive was forwarded to Moscow last December by the Russian embassy in Washington. Six months passed before Korda received a call to discuss the project. Then he met with a Russian named Kowalsky. Despite Korda's willingness to pay an advance - the sutomary way of doing business - the Russians preferred to complete the manuscript before talking price.

Korda then wrote a longer letter with suggestions and, sure enough, several months later, he received a call telling him the manuscript was completed. And translated. And neatly typed.

Korda guesses that Kowalsky was head of a team of researchers at the Academy of Sciences of the U.S.S.R.Such a team was apparently responsible for putting together the book titled Leonid I. Brezhnev, Pages From His Life.

Three days after he was handed the book, a team of Russian negotiators arrived. Korda obtained world rights (except for Socialist countries) for "a considerable sum" after earnest bargaining.

"When we did our contract it was thirty or forty pages of small type," Korda recalls. "They had done enough homework to know what was in it and what wasn't and over a weekend they translated the whole contract into Russian. They were fluent on the subjects of indemnity and warranty clauses, bankruptcy of publisher clauses."

Korda says he's not bothered by either the mystery surrounding the book or the team approach to the research and writing.

"I guess they're admitting to something most American heads of state won't admit," argues Korda. "Richard Nixon, for example, probably has twelve people involved with writing his book. And it's more honest than to pretend that Ike spent his mornings at the typewriter like Proust staring into space waiting for the perfect sentence. When in fact he spent his day playing golf and when he got home Joe Barnes (ghostwriter of an Eisenhower book) would say 'General, here's the next five pages, how do you like them?'"

Korda will learn how Americans like the 100,000-word Brezhnev book next spring; publication is set for April. The advance, incidentally, was paid to Soviet Life, the magazine published by the Russians for Americans.