Princess Diana’s brother wants to the world to know the truth about his sister.

Earl Charles Spencer, 62, announced Aug. 18, that his book “Swan Song: Diana, My Sister,” will be released in the United Kingdom next month. The book marks the first time he “has spoken openly and at length” about his older sister, according to publisher Penguin Michael Joseph, a division of Penguin Random House.

“Beautiful. Beloved. Hunted,” Penguin writes of the late princess, adding that Spencer documents her “extraordinary life and death,” through his writing.

“I want to speak on Diana’s behalf and do so in an openly positive way,” Spencer said of his decision to write the book, which will be published globally and translated into 12 languages. “The aim of this book is to tell the truth about Diana from her brother’s perspective,” he said.

Spencer revealed that he has long been asked to pen a book about his late sister and feels that now is the right time to share his story. Next year will mark three decades since she was killed in a car crash in Paris in 1997, leaving behind her two sons Prince William and Prince Harry.

“With the 30th anniversary of Diana’s tragic death on the skyline, I’ve been inundated yet again with requests for interviews about my sister,” Spencer said. “This has convinced me to write down my own thoughts and memories, once and for all, rather than having the discomfort of again reading the opinions of others, many of which are based on untruths that have become accepted over time.”

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Spencer, an author and historian, said his hope is for those reading his words to come away an understanding of who his sister was: “a one-off, dynamic character, full of love, charisma and self-doubt, who gave life a hell of a go.”

Daniel Bunyard, publishing director at Penguin Random House UK, described Spencer’s book as “extraordinarily moving, beautifully written and remarkably candid.”

Bunyard noted that the novel, slated for release on Sept. 22, would be one of “historic importance.”

Last month, the late princess would have marked her 65th birthday.

Earlier this year, to mark Mother’s Day in the United Kingdom, Prince William shared an unseen photo of his mother, alongside the caption “Remembering my mother, today and every day.” The photo, taken in 1984, shows a young Princess Diana with a 2-year-old William in a field of flowers.