Charles Spencer is to publish about a book about his sister Diana in September.

In the book – titled Swan Song: Diana, My Sister – Spencer recounts the story of Diana’s life and the week of her death. It marks “the first time that Earl Spencer has spoken openly and at length about his sister”, according to the book’s publisher, the Penguin imprint Michael Joseph. The book will be released on 22 September in the UK.

Photograph: Penguin

“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,” said Spencer. “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.”

“The aim of this book is to tell the truth about Diana from her brother’s perspective, just as I attempted to do when speaking at her funeral. As with my eulogy, I want to speak on Diana’s behalf and do so in an openly positive way.”

Spencer delivered his sister’s eulogy at Westminster Abbey in September 1997, after her death on 31 August.

Daniel Bunyard, publishing director at Penguin Michael Joseph, said the book contained “many revelations that will draw considerable attention”, but was also “a fitting tribute to the beloved woman who was Princess Diana”.

Spencer said: “I hope this book will be of interest to the many who still cherish the memory of Diana when she was alive.

“I would be delighted if those too young to remember her in her heyday feel, after reading this, that they ‘get’ who she really was: a one-off, dynamic character, full of love, charisma and self-doubt, who gave life a hell of a go, and left the world a far better place, despite exiting the stage while still so young.”