Spencer has said his purpose in writing Swan Song is to record his own memories before the 30th anniversary of Diana’s death.
He said: “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.”
Spencer added: “I hope this book will be of interest to the many who still cherish the memory of Diana when she was alive.”
Diana became one of the world’s most recognizable public figures after marrying Prince Charles in 1981. The couple had two sons, Prince William and Harry, before separating in 1992 and divorcing in 1996.
Diana became celebrated for her humanitarian work, notably campaigning against landmines and challenging stigma surrounding HIV and AIDS.
Her turbulent private life and relationship with the royal family also attracted intense global scrutiny.
Diana was 36 when she and Dodi Fayed, 42, were killed following a car crash in the Pont de l’Alma tunnel in Paris on August 31, 1997, prompting worldwide mourning.