LONDON, Jan. 5 (Xinhua) -- Rare letters written by the late Princess Diana of Wales mentioning her two sons were sold by auction Thursday for well over their expected price.

The sale at the Cheffins auction house in Cambridge attracted world-wide interest.

In one of the letters Princess Diana described how her eldest son William, second in line to the British throne, was "constantly in trouble" at his boarding school.

The princess, first wife of Prince Charles, eldest son of Queen Elizabeth II, handwrote the letters to a confidante Cyril Dickman who worked at Buckingham Palace for more than 50 years as a steward.

In another letter, Diana described how Prince William could not stop kissing his newly arrived brother Prince Harry after his birth in 1984.

That letter was sold for 4,000 U.S. dollars, while the one referring to the troublesome William fetched 3,000 U.S. dollars.

Diana wrote: "William adores his little brother and spends the entire time swamping Harry with an endless supply of hugs and kisses, hardly letting the parents near."

In another letter Diana reveals the naughty side of Prince Harry, who was then aged eight. In her letter to Dickman, she wrote: "The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!"

She added that she and Prince Charles were later that year due to visit Korea adding it would be a good place to do the Christmas shopping.

Dickman started his career at Buckingham Palace during the Second World War, watching out for fires on the roof during the German Blitz on London, becoming head steward at the Royal family's official residence. It seems Princess Diana held him in high esteem, hence the flow of letters about her two royal sons.

Dickman retired at 65 and was made a Freeman of the City of London. After he died aged 85 in 2012, his grandson uncovered the collection of letters and greetings cards sent to him by members of the Royal family.

Other items sold included collections of silver spoons, coins and cufflinks given as gifts to Dickman by the Royal family.

In total the 40 lots of items in the collection of memorabilia from Dickman's collection were sold for 68,300 U.S. dollars, well exceeding an estimate of 16,100 U.S. dollars for the collection.

Dylan Mander, specialist at Cambridge-based auctioneers Cheffins which auctioned off Dickman's letters, said: "It is incredibly rare to have letters from Princess Diana with such a personal touch to them."

Princess Diana was killed in a traffic road accident in Paris on Aug. 31, 1997. By then she had parted from Prince Charles.