The book, "A Swim-On Part in the Goldfish Bowl: A Memoir," tells of Carol's shock when her now 82-year-old mother's "blotting-paper brain" became confused between Bosnia and the Falklands during a conversation about the war in the former Yugoslavia, The Sunday Times of London reported.
"I almost fell off my chair. Watching her struggle with her words and her memory, I couldn't believe it. She was in her 75th year but I had always thought of her as ageless, timeless and 100 percent cast-iron damage-proof," Carol Thatcher wrote.
She also tells of having to repeatedly break the "truly awful" news of the death of her father Sir Denis to her ailing mother until the information sank in.
The book marks the first time the Thatcher family has spoken publicly about the former Conservative prime minister's mental slide.
"Whereas previously you would never have had to say anything to her twice, because she'd already filed it away in her formidable memory bank, Mum started asking the same questions over and over again, unaware she was doing so," Carol wrote of her mother.
Thatcher, who headed the British government between 1979 and 1990, stopped making public speeches after a series of small strokes.