Lady Susan. 83, was part of the team of palace officials asked to help Meghan adjust to royal life, Bower writes.
He also claims in the book that there was bad blood between Meghan and Victoria Beckham, amid suspicions that the former Spice Girl was leaking stories about her to the media.
Harry reportedly even confronted her husband David Beckham about it, Bower claims.
He writes: "Harry and Meghan were shackled.
"Every night they scoured the internet to read the newspaper reports and the trolls' postings on social media.
"Irrationally, they grouped the two together and fed each other's frenzy about the media.
"Convinced that as champions of goodness they were being persecuted by mendacity and racism, they felt victimised by the mildest criticism.
"Harry's instability fed Meghan's fears that friends were leaking stories to the media.
"In particular, she suspected Victoria Beckham of indiscretion.
"Harry called David Beckham to repeat the accusation. Outraged, Beckham's truthful denials damaged their relationship."