IN THE late 1940s, Cynthia Jeffries was 22 and living in Paris when she answered a small newspaper advertisement for temporary secretary, placed by the celebrated author Arthur Koestler.

She got the job and for the rest of her life, friends say, was deeply devoted to him.

Eventually they married. Last week Cynthia Koestler was found dead with her husband in their London apartment. Notes made it clear they had committed suicide. Koestler was 77, desperately ill with Parkinson's disease and leukemia. As a longtime advocate of euthanasia, his death had a kind of logic.

But Cynthia Koestler was 56, in good health, a woman who might still have had many full and productive years. The obituaries acclaimed her husband's considerable achievements in prose and fiction and mentioned her only in passing.

"He married his third and last wife, Cynthia Jeffries, in 1965," the London Times noted. "His wife was found dead with him."

What kind of person, then, was she? Why did she choose this final act?

"Arthur was her whole world," said Joan Lee Thompson, a writer who knew her for 30 years. "After he made his decision, she would not have hesitated in joining. He would never have to sway her. She would do it herself. For all her shyness, Cynthia had a very strong will."

"I suppose her death raises Cynthia's love to a tragic level," said Celia Goodman, another longtime friend, "but not one minute of the day would she have been happy without him. He always called Cynthia, 'angel,' and she was as near an angel as a person can be."

Judging from the tributes of those who knew them best, the Koestlers' relationship was not terribly complicated. She revered him and became, as the years progressed, an inextricable part of his life. "His companion, his colleague, his collaborator," said Pat Kavanaugh, Koestler's literary agent. They were working on a concluding volume of his autobiography together, she said.

Not long ago, Cynthia Koestler completed her first chapter. It was titled: "How Unpleasant to Meet Mr. Koestler."

Cynthia Jeffries was born in South Africa, Jewish on her mother's side and Irish on her father's. About the time she was 10, said Goodman, she heard the crack of a gunshot when her father committed suicide. She was sent to school in England and then moved to Paris to work on her French. She saw Koestler's advertisement. He was living near Fontainebleau and she went to work there.

In 1950, Koestler, recently married to his second wife, moved to New Jersey and sent for Cynthia Jeffries. Cynthia moved in with the couple, but only became his lover after his move to Britain, his second divorce and, Kavanaugh said, a brief marriage of her own. It was another decade before they married. Even before that, however, she officially adopted his name.

Cynthia, small and unusually youthful in manner and appearance even into her fifties, was an energetic and efficient assistant. Although she was shy and somewhat retiring, friends said, she became a hospitable partner for Koestler, who enjoyed entertaining. His friends were her friends. She apparently had few of her own.

"She was very humble and self-effacing," said Goodman, who is the sister of Koestler's second wife, Mamaime Paget. "She'd get up at 6:30, did all the driving, shopping and paperwork. She typed every page. At the end of his life she nursed him . . . She lived for Arthur."

Her two hobbies were dogs and gardening at their country house near Cambridge. A month ago, with Koestler already seriously ill, she told Kavanaugh she was losing interest in gardens, which in retrospect, may have been a function of her decision not to go on without him. A few days before her death, she took a beloved Lhasa apso named David to the vet to be put down but told the maid the dog had been given away.

Thompson thinks the joint decision to commit suicide was made about six weeks ago, although other friends suspect it might have come more suddenly than that. Cynthia was arranging for friends to come to dine as recently as the week before last. On Feb. 28, the day before the couple probably died, she called one woman to postpone a visit and sounded agitated. The same day milk and newspaper deliveries were canceled.

The Koestlers' choice to take their lives was, friends believe, completely rational. Both were members of an organization called EXIT: The Society for the Right to Die With Dignity, and Koestler had written the forward to a controversial EXIT booklet several years ago that offered advice on quick and painless means of taking one's own life. It was called, "A Guide to Self Deliverance."

As her responsibility to care for Koestler increased, Cynthia was under considerable strain, but aside from taking sleeping pills at times, she seemed able to cope. Some accounts have described him as quarrelsome and difficult and portrayed her, as one acquaintance put it, as his "doormat."

That is not an accurate picture, friends insist. While his health failed, they say, his mind did not, and he treated Cynthia with tenderness and respect.

A year ago, when it began to be obvious that Koestler's illnesses were increasingly debilitating, Goodman proposed to Cynthia that after his death, rather than live alone, she should come to live with her.

"She was pleased at the time," Goodman recalled, "and then as the months went on, I could tell she was losing interest."