Rumer Willis Has Already Read Mom Demi Moore's Memoir -- Here's Her Review! (Exclusive)

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Rumer Willis is "so incredibly proud" of her mother, Demi Moore.

The 56-year-old actress will be releasing her "deeply candid" memoir, Inside Out, in September, and her oldest daughter has already read it. "I actually finished it not too recently and I'm so incredibly proud of her," Rumer told ET's Keltie Knight at the Los Angeles premiere of Once Upon a Time in Hollywood on Monday. "It's so beautiful and she shows such courage and vulnerability."

Rumer further praised her mother's upcoming book, boasting, "I think [it shows] a depth inside of herself that no one has really seen yet, which I'm really excited for people to get to know that side of her."

According to a press release from HarperCollins, Demi doesn’t hold back discussing the ups and downs she faced on her rise to fame and is forthright when talking about her battles with addiction and body image.

The Ghost star also opens up in the book about her three high-profile marriages. She was married to musician Freddy Moore from 1980 to 1985. In 1987, she married Bruce Willis, and the two have three daughters, 30-year-old Rumer, 28-year-old Scout and 25-year-old Tallulah. Demi and Bruce divorced in 2000, and five years later, she married Ashton Kutcher, who is 16 years her junior. The two divorced in 2013.

Inside Out is first and foremost a woman’s story; that the woman in question happens to be one of the most celebrated actresses of our time only makes her journey of vulnerability, strength, and self-acceptance all that more resonant,” HarperCollins’ Jennifer Barth said in a statement. “I think readers are going to be surprised -- and moved -- by this book, which Harper is extremely proud to publish.”

Demi's memoir is to be released on Sept. 24.

Scout Willis, Tallulah Willis and Rumer Willis
Axelle/Bauer-Griffin/FilmMagic

Meanwhile, Rumer made the Once Upon a Time in Hollywood premiere a family affair, showing up with her two sisters to the event. She also shared with ET the moment she heard she had landed a part in Quentin Tarantino's highly anticipated movie.

"It was three days before my 30th birthday, so that was the best birthday present that I could have ever gotten," she said. "I think I shot maybe two days after and honestly there's a part of it, I was talking to my sisters, that's so fun for me because it feels like this lineage because my dad was in it, you know what I mean?"

Rumer, whose dad, Bruce, starred in Tarantino's Pulp Fiction, further quipped, "I literally would have been a background actor if he asked me to. I think it's so awesome."

Once Upon a Time in Hollywood hits theaters on July 26.

Here's more with the famous family.

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