"They were part of the same movement, same values, same commitment. And I think what he was beginning to realize there is that 'Her struggle, her personal journey in that broader struggle is a tougher one than mine. She's exposed to more than I am. She's paying a heavier price than I am,'" Harris shared with ABC News.
After Winnie was detained in 1969 and placed in solitary confinement for 17 months, Mandela wrote to his daughters Zeni and Zindzi.
In a note dated June 23, 1969 he penned: "Once again our beloved Mummy has been arrested and now she and Daddy are away in jail. My heart bleeds as I think of her sitting in some police cell far away from home, perhaps alone and without anybody to talk to, and with nothing to read. Twenty-four hours of the day longing for her little ones."
Harris said that passage from Mandela's writing showed what he was enduring.
"What emerges that is perhaps fresh is the level of pain that he was carrying while he was in prison as he saw her life from his perspective unraveling, and particularly the impact of her incarceration and torture," Harris said.
Mandela was in prison when his oldest son Thembi died in an accident. In a letter written from Robben Island Prison in July of 1969, Mandela appealed to the commanding officer writing, "I should be pleased if you would give me permission to proceed immediately,with or without escort, to the place where he will be laid to rest. If he will already have been buried by the time you receive this application, then I would ask that I be allowed to visit his grave for the purpose of 'laying the stone', the traditional ceremony reserved for those persons who miss the actual burial."
Mandela's request was denied.
"It froze his heart," Harris told ABC News. Mandela's mother had died only 10 months earlier and his plea to attend her funeral was also denied.
As Harris sifted through Mandela's taped conversations, unfinished manuscripts, scribbled notes, and carefully crafted letters for "Conversations with Myself," he noticed a personal discussion Mandela has with himself over the years – reconciling the "burden" of his public image.
"He is the chosen one. And it really started in a focused systematic way in the 1970s. He's imprisoned. The ANC decides to use him as a symbol of struggle -- the Free Mandela campaign, and so on. And he carries then a huge burden of responsibility. Everything he does, his private life impacts directly on that public representation," Harris said.
"This is a fallible human being aware of that fallibility and increasingly uncomfortable with the way in which that fallibility has been hidden publicly."
"We can all aspire to that. We can all relate to it. I don't think you can relate to the saint or the icon, the larger than life leader. But you can relate to this Nelson Mandela. And that's what we're hoping to share in the book."