The insider also told the publication that the word ‘Megxit’ in particular, referring to the Duke and Duchess' decision has "always angered" Prince Harry.
"It gives the impression that the decision to walk away from the Royal Family was Meghan’s," they explained.
“The reality is Harry drove that decision. The book will make that clear and explain why it had to happen. The truth is Harry had been unhappy for a long, long time."
In the year it was revealed Harry and Meghan had started dating, the red-headed royal released a statement defending his then girlfriend that reflected his unhappiness with the constant media attention.
"Prince Harry is worried about Ms. Markle’s safety and is deeply disappointed that he has not been able to protect her. It is not right that a few months into a relationship with him that Ms. Markle should be subjected to such a storm," the statement read.
"He knows commentators will say this is ‘the price she has to pay’ and that ‘this is all part of the game’. He strongly disagrees. This is not a game - it is her life and his."
Since their big move to LA, the Duke and Duchess of Sussex's life hasn't exactly gone to plan.
Royal editor Russell Myers revealed in an interview with Radio New Zealand that “all their plans are simply up in the air at the moment."
"They had huge plans once they settled in Los Angeles... to set up their own foundation and now obviously these plans are in tatters."
He added: "They had plans to launch themselves into the corporate world, into the very, very lucrative world of public speaking, where they could potentially net millions and millions of pounds for a single gig."
Russell also remarked that whilst Meghan is "very adaptable", Harry still needs to adjust to their new life.
"She's a successful actress, she's had success speaking with the UN, speaking on a wide range of charitable endeavours.
"They could be a tour de force as a collective, but Harry really needs to find his feet.”