The memoir has generated controversy elsewhere.
Spencer has apologized to Piers Morgan, 61, after acknowledging he had made an incorrect assumption in the book concerning Morgan and intrusive photographs of Diana.
Morgan has threatened legal action and demanded damages.
Spencer says he wrote his 384-page memoir ahead of the 30th anniversary of his sister’s death because he wanted to record his memories rather than repeatedly encounter what he regards as inaccurate accounts of Diana’s life.
Another royal source said: “There is inevitably suspicion about Harry’s influence on Charles’ book because of the timing, his closeness to Charles and the fact that security has become such an important issue for him and Meghan again.
“Charles invoking what happened to Diana while arguing that Harry and his family require greater protection is enormously powerful. Whatever his intentions, it reinforces one of Harry’s central arguments – that nobody should wait for something terrible to happen before taking the threat seriously.
“Harry and Charles are close, and the Sussexes spent time at Althorp over the summer, so naturally people are wondering what conversations took place privately. That doesn’t mean Harry dictated a single word of the book, and Charles insists he didn’t.
“But the themes overlap in a striking way. Diana’s treatment, the consequences of relentless attention and Harry’s concerns about protecting Meghan and their children are all part of the same emotionally charged territory.”