"On bad days, she could hardly remember the beginning of a sentence by the time she got to the end," she writes. "I had to learn to be patient, a quality I admit is in short supply. I also had to learn she had an illness and that it wasn't personal. That's the worst thing about dementia: It gets you every time."
Reagan's youngest son offered his condolences on Thatcher's now-public condition and agreed the insidious disease can strike anyone, regardless of their mental acumen.
"The disease doesn't respect your mental faculties any more than any other disease, like cancer or the flu," said Reagan, now a political commentator for MSNBC.
Still, Reagan, whose family only painted a positive face of the once-hearty former president, was critical of Carol Thatcher.
"What exactly does the public learn when we hear she doesn't recognize her family?" he asked. "Everyone knows what it does and what happens: You forget names, places, things. To detail that is shameful."
"People should think about how they would feel if it were their mother," Reagan said of the intimate details of Thatcher's mental decline. "The word is 'salacious.'"
Amanda Platell of Britain's Daily Mail, which is serializing the memoir, agreed.
"It felt not only like a terrible invasion of an old woman's privacy, but a personal betrayal," she wrote. "There's a time and place for such memoirs. But this was not it. Too soon, Carol. Too much detail."