Diana was 36 when she died following a high-speed car crash in Paris on August 31, 1997, a year after her divorce from Charles.
Spencer’s memoir, due to be published on September 22, revisits Diana’s childhood, marriage and death, as well as the traumatic preparations for her funeral.
Harry, now 42, has previously described the decision to have the boys walk behind their mother’s coffin as “barbaric” in Spare.
He wrote of the procession: “I remember feeling numb and clenching my fists. I remember keeping a little piece of Willy in the corner of my eye because it gave me strength.”
William, now 44, reflected on the experience in 2017.
He said: “It was one of the hardest things I’ve ever done, that walk. It felt she (Diana) was almost walking along beside us to get us through it.”