"Finding Freedom: Harry and Meghan and the Making of A Modern Family" details the life of Harry, 35, and Markle, 39, from the early days of their romance up until their "step back" from their duties as members of the royal family.
The book claims that the prince and his friend Tom "Skippy" Inskip, whom he met during his time at Eton College, had a falling out after Inskip explained that he "had doubts" about his relationship with Markle, per Daily Mail.
Inskip urged Harry and Meghan to live together before "doing anything more serious," the authors were reportedly told.
Despite Inskip's advice coming "from a good place," Harry "didn’t totally see it that way,"
"It really hurt him that someone he was so close to would not trust his judgment," the authors were told by a source that is said to have gotten permission from the Duke and Duchess of Sussex to open up about the relationship.
Inskip and his wife were "punished" for what Inskip said by being left out of the royal pair's wedding party at Frogmore House, while other stars like George Clooney and Oprah Winfrey were reportedly in attendance.
The prince is believed to have reconciled with his friend after Inskip's mother-in-law passed away last year.
Inskip isn't the only person close to Harry to have given such advice, the book claims.
Per The Hollywood Reporter, William told his little brother to "take as much time as you need to get to know this girl," which upset Harry.
Additionally, William feared that Harry was "blindsided by lust."