Omid Scobie, who co-wrote Finding Freedom with Carolyn Durand, has criticised media coverage of the book as "off the mark".
Several extracts have been published ahead of the book's publication on August 11, detailing simmering tensions and explosive arguments among members of the Royal Family.
Some have called Finding Freedom a "bloodbath" — but Scobie has dismissed this perception of the book and said he intended to "humanise" Meghan after years of unflattering coverage.
"We are really used to seeing royal sources and palace sources in stories about the duke and duchess," he told the On Heir podcast.
"It was very clear to many that those in their lives were increasingly frustrated by the treatment of the couple.
"We feel it [Finding Freedom] really accurately depicts what went on.
"It takes you from the first days they met."
Harry and Meghan have publicly stated they had no involvement with the book, despite some very personal stories making their way onto the pages.
Scobie has emphasised Finding Freedom is "unauthorised and unofficial", and he relied on his own experience as a member of the royal press pack as well as interviews with close sources.
"I spent time with Prince Harry even before he and Meghan were married," he said.
"I hope it [the book] humanises the Duchess of Sussex," he added.
"[It's] a beautiful love story [and] against the odds allows them to thrive in a space where they weren't thriving before."
He said many people who have speculated on what the book will reveal are "off the mark", as well as reports that Finding Freedom paints an unflattering picture of the Sussexes in their split with the Royal Family.
Reporting around the book has been "unbalanced", he added, with many outlets taking a negative view of Harry and Meghan's behaviour and decisions.
Instead the book depicts a well-intentioned couple who became frustrated at senior royals, including Prince Charles, refusing to discuss the possibility of them stepping down.
Harry and Meghan even considered driving straight to see the Queen to plead their case after returning from Canada in January, the book claims.
Finding Freedom quotes a heartbroken Meghan as saying: "I gave up my entire life for this family. I was willing to do whatever it takes. But here we are. It's very sad."