The unpublished book says Charles was quite interested in Williams' brushes and tubes of paint when he and his kid sister, Princess Anne, visited the artist's London studio for their early official portraits.
"Charles was less interested in the toys than I had expected him to be," Williams wrote. "He has a practical turn of mind like his father, and it was my activities with the brushes and paints that most attracted him."
The prince has carried his interest in painting on into adulthood and is known as a fairly accomplished water colorist.
Charles and Anne were sent to Williams' studio in the early 1950s when he was 4 and Anne was just 2. Charles began showing up with his own paint set and book after only a few visits.
The Daily Telegraph said Saturday that Charles was an enthusiastic artist, although Williams, who died in 1960, was diplomatic about her young subject's final product. "The bold lines and strokes and no less lively use of color were certainly imaginative and suggested the very modern school," she wrote.
"When his first picture was finished -- and I must say that it was produced with great deliberation -- I asked what it represented," Williams wrote. "Without hesitating for a moment the young prince replied: 'Oh, it's a forest.'"
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