Warrior: Audrey Hepburn will be published by GoodKnight Books on Sept. 28
Audrey Hepburn
Audrey Hepburn on a humanitarian trip
| Credit: Derek Hudson/Getty

Audrey Hepburn is known as the dark-eyed darling of the Golden Age of Hollywood who later championed humanitarian work. But there is much more to the second part of her legacy than has been told before — until now.

Robert Matzen, author of the 2019 bestselling biography Dutch Girl: Audrey Hepburn and WWII, has penned a new book about the actress' dangerous work for UNICEF and the last five years of her life before her death at the age of 63 to colon cancer. Warrior: Audrey Hepburn will be published by GoodKnight Books on Sept. 28 and includes a moving introduction by Hepburn's younger son, Luca Dotti.

"There are hundreds of books about my mother and each in its own way tries to solve a piece of the 'Audrey puzzle,' " Dotti, 51, tells PEOPLE via email. "Some are instant classics while others are merely exploitative, and until now none has focused on what she considered her 'most important career' — her fight for children in need in what she called the Developing World, often in war zones.

"The stories she brought back home and shared with us were chilling but always sugarcoated until Warrior totally challenged my perspective," he continues. "Mum was more than a steel-butterfly; she was a battle-hardened badass, and Warrior: Audrey Hepburn finally tells that story."

Audrey Hepburn book
Credit: GoodKnight Books

Hepburn, who nearly died of starvation as a teenager in Holland during World War II, became a film icon with her starring roles in films like Funny Face and Breakfast at Tiffany's. She was only 38 when she decided to raise her two sons — Sean Hepburn Ferrer, now 60, whom she shared with ex-husband Mel Ferrer, and Dotti, whom she shared with ex-husband Andrea Dotti — quietly in Switzerland. Ten years after that life-changing decision, Hepburn became an ambassador for UNICEF, the same organization that had helped her during the war. She pursued her life's work until her death in 1993.

Hepburn was willing to do more than smile sweetly in front of cameras to help people suffering across the world.

"UNICEF expected that Audrey Hepburn would be a pretty princess for them at galas," Dotti said, according to the book's press release. "What they really got was a badass soldier."

Hepburn went on multiple trips to dangerous areas across the globe, facing bombs and bullets. She witnessed "hell on earth" in Somalia, according to the press release. In Warrior, Matzen also delves into Hepburn's final years.

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"Like Dutch Girl, Warrior tells the story of an unexpected Audrey. So many people have said to me, 'Oh, I love Audrey,' without being able to put into words exactly why," Matzen explains via email. "Warrior provides some answers because this is a down-to-earth, funny and fearless Audrey Hepburn, this too-thin waif of a woman charging into war zones at age 60 and beyond, standing in front of cameras with starving children and mothers not to publicize herself, but to bring the world's attention to life-and-death situations."

After intensive research and interviews with the people who loved her best, including Dotti, Matzen's view of the beautiful icon was forever changed.

"I went into these books neutral about Audrey Hepburn and now I can say without hesitation, I love this woman," says Matzen. "Warrior is the story of a genuine hero who signed on to UNICEF and then drove herself mercilessly for the cause.

"The crazy thing is, this story was almost lost to history because no one had documented these missions in detail," he continues. "Somalia in particular is gut-wrenching as Audrey 'went to hell' as she phrased it and while there she stunned seasoned U.S. military men with her courage under fire."

* With reporting by LIZ MCNEIL