Miss D and Me: Life with the Invincible Bette Davis (available Sept. 12) is excerpted exclusively in PEOPLE
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Bette Davis was mother to two daughters and a son, but in the final decade of her life she welcomed a surrogate daughter into her life.

“She taught me so much,” says Kathryn Sermak, who served as the prickly two-time Oscar winner’s personal assistant off-and-on from 1979 until Davis’s death in 1989.

Sermak was just 23 when she interviewed to become the All About Eve star’s live-in assistant.

“Miss Davis seized my hand firmly, increasing her grip. She left an imprint on my fingers. “How do you do?” she said in her honey-and-gravel voice,” Sermak, now 61, writes in her memoir, Miss D and Me: Life with the Invincible Bette Davis, excerpted exclusively in the current issue of PEOPLE.

In her first days as Davis’s assistant, Sermak’s handshake became the topic of much discussion.

“I’d like to speak with you about your handshake,” Miss Davis said not long after I started. “You can tell a worthwhile person by their handshake. I would like yours to be a bit firmer.”

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  • For more from Sermak — including how Davis reacted to the scathing tell-all that was written by the star’s daughter — pick up the latest issue of PEOPLE, on newsstands Friday

Over the years, Sermak and Davis grew close. The two traveled and stayed together any time Davis vacationed or was on location for a shoot. In 1983, Davis, then 75, suffered a massive stroke. Sermak stayed in the hospital with her for three months. In her final years, Davis had Sermak accompany her on a few European vacations. During one of them, the star asked Sermak to make a “pact” with her.

“Whichever of us dies first, the other will see to it that we look beautiful at the last,” Sermak recalls Davis saying.

Davis also encouraged Sermak to write.

“Miss D said, ‘You’ve got to tell our great story. You can set the record straight,” says the former assistant.

RELATED VIDEO: How Ryan Murphy’s Early Meeting with Bette Davis Inspired Feud

Now, in Miss D & Me, Sermak recounts Davis’s final days, which came in 1989. They were on vacation in Europe — where Davis had been — when the actress was hospitalized and told by doctors that her breast cancer, in remission for five years, had “exploded,” and she had just days to live.

“Miss D accepted it,” Sermak writes. “She told me to call [her son] Michael and then [her lawyer and friend] Harold. She did not ask me to reach out to Bede[, her daughter with whom she had a tumultuous relationship.] She had her moments with them over the phone, comforting those she loved so deeply. She did not want them to come because she did not want ‘this bedraggled body and the look of death’ to be their last memory of her. She was not worried that I would have the same trouble. ‘They don’t see me every day like you do, Kath,’ she said.”

On Oct. 6, 1989, Sermak held Davis’s hand During her final moments.

“I squeezed her hand, hoping she would squeeze back as she had those days
in the hospital after the stroke, but I got no response,” Sermak writes.
“At 11:35 p.m. she no longer inhabited her body but her spirit was all
around me.”

Miss D and Me: Life with the Invincible Bette Davis is available Sept. 12.