"My dear darling, I'm just writing you a very little letter," Shawcross read from one of their correspondences. "I should be thinking about you when you get this, and hoping that everything will go off wonderfully well. I'm sure it will. Also, I might add that I do love you, Bertie, and feel certain that I shall more and more. I shall miss you terribly. You are such an angel to me."
"They grew together in the most beautiful way," Shawcross said.
Albert was not in line to become king. Instead, his brother David was. But David famously fell in love with American divorcee Wallace Simpson and gave up his title to marry her.
David's abdication meant Albert was in line to be king and Elizabeth to be queen.
"He [Albert] didn't feel equipped for it, and quite frankly, if he hadn't been married to Elizabeth, he would have found the job almost impossible, because she gave him strength and such confidence," Shawcross said.
When Albert, then known as King George, died at 56 years old in 1952, the Queen Mother settled into a new life as a widow.
"The death of the king in 1952 was a catastrophe for the queen," Shawcross said. "She was only 51 herself, and as it happened, she lived another half of her life -- another 50 years -- as a widow."
Throughout the rest of her life, she spent months at a time at Balmoral -- a place she felt connected to.
"She loved the country. She enjoyed walking a lot. She enjoyed fishing, and later in life, she was fishing in waders, right up into her 90s," Shawcross said.
Being at Balmoral wasn't just good for the queen, it was good for the kingdom, Shawcross said.
"I think the fact that the royal family loved Scotland so much, since the early part of Queen Victoria's reign, has had a very important effect on the unity of Scotland and England. The royal family's love of Scotland has helped keep those parts of the kingdom together," Shawcross said.
At a nearby neighborhood pub, patrons remember the queen interacting with people in the town when she came in for shopping or to visit the butcher.
"You don't expect people like that will talk to commoners, but she did," one patron told "Good Morning America."
"She earned a lot of respect," said another.
More than respect, Shawcross said Elizabeth earned the love of her people and carried on the royal tradition -- the one that dates back to the times of knights and damsels in distress.
"She was dependable and loyal and utterly reliable," Shawcross said. "She represented that sense of the golden thread of history, in a marvelous way."
ABC News' Lee Ferran contributed to this report.