Einstein's personal life was marked by a streak of callousness and aloofness, Dr. Schulmann said. Nowhere is that more evident than in the strange letter, dated May 25, 1918, that Ilse Einstein sent to her close friend Georg Nicolai, a doctor, well-known antiwar crusader and Einstein family friend, with the words, ''Please destroy this letter immediately after reading it'' scrawled across the top in big letters. At the time Einstein, who had moved to Berlin four years earlier to take a lucrative appointment to the Prussian Academy of Sciences and become director of the newly formed Kaiser Wilhelm Institute of Physics, was 38 years old and in the process of divorcing Mileva, presumably so that he could marry his cousin Elsa. Ilse, the older of Elsa's two daughters, was serving as Einstein's secretary.

The letter was a plea for advice. In it Ilse related how a simple ''jest'' one afternoon had suddenly escalated into a serious proposal that Einstein marry her instead of her mother. Einstein, she said, had confessed that he loved her. Moreover her mother, she reported, was prepared to step aside, if that was what would make Ilse happy.

''Albert himself is refusing to take any decision; he is prepared to marry either Mama or me,'' Ilse wrote. ''I know that A. loves me very much, perhaps more than any other man ever will, he also told me so himself yesterday,'' she went on.

The feelings, however, were not quite reciprocal. Ilse loved Einstein like a father, she wrote, but she had no desire to be close to him physically. Her instinct, she confessed, was not to marry him. ''It will seem peculiar to you that I, a silly little thing of a 20-year-old, should have to decide on such a serious matter; I can hardly believe it myself and feel very unhappy doing so as well. Help me!''

There is no evidence that the relationship with Ilse was ever consummated. Einstein and Elsa were married the next year and remained husband and wife until her death in 1936. (Ilse later married Rudolf Kayser, a writer and literary critic who subsequently wrote a biography of Einstein. She died of tuberculosis in 1933.)

Ilse's letter first came to the attention of the Einstein scholars in the early 1990's. At least one of the other editors, Dr. Schulmann said, thought the letter might be a product of Ilse's imagination. ''I take it very seriously,'' said Dr. Schulmann, noting that Nicolai was a former lover of Ilse, as well as an older man, in whom it would be natural for her to confide.

He said he saw the letter and the situation it described as an example of Einstein's vaunted ability to detach himself from the people around him. ''The most telling point is that he doesn't care,'' Dr. Schulmann said. ''That's the Einstein we know. He is indifferent to the question of whether he marries one or the other.''

Ilse and her confidant Nicolai also emerge in the new volume as key influences on Einstein's political activities during the war. Shortly after the outbreak of hostilities, Einstein was one of only three people to sign an ''Appeal to Europeans'' deploring the destruction of international culture, making him a hero to the pacifist movement, and he attended meetings of a pacifist political club, the Bund Neues Vaterland. Historians have long thought that Einstein drew up the manifesto, but the evidence, Dr. Schulmann said, suggests that it was actually written by Nicolai, an antiwar crusader, and author of a book called ''The Biology of War.''

But, again, detachment characterized Einstein's actions. At the same time that he was attending antiwar meetings, presumably at the behest of Ilse and Elsa, Dr. Schulmann said, Einstein consulted for a maker of submarine gyroscopes and even tried to design an airplane for the military. In his letters he often said that the war did not intrude on him. ''Why shouldn't one let the servants live happily in the madhouse?'' he wrote to a Swiss friend.