The correspondence - handwritten on headed Kensington Palace paper - was sent to ex-Buckingham Palace steward Cyril Dickman, and is due to be sold as part of his estate.
Now previously unseen candid letters, written by Princess Diana, have surfaced.
Letters penned by Britain's late Princess Diana in the 1980s and 1990s were sold for a sum far exceeding their estimated value at an auction on Wednesday in Cambridge.
Diana said it was a "particularly happy time" in a letter sent five days after Harry's birth in September 1984.
Among the letters was a note appearing to wish Mr Dickman well on his retirement in 1992.
She added: "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!"
A letter Diana wrote describing the immediate bond between Prince William, now 34, and his younger brother, Harry, now 32, sold for almost $4,000.
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The touching letters will be sold by Cambridgeshire auction house Cheffins on January 5th.
They reveal personal family insights such as how Harry was "constantly in trouble" and how much the boys enjoy boarding school, reports ITV News.
That letter sold for almost $3,000, well above of its estimated approximately $900 selling price.
There are around 40 lots in the collection, including letters and cards from other members of the royal family, including Prince Charles, and a piece of boxed cake from the Queen's wedding in 1947.
The lot containing the revealing letter sold for £2,400, more than double the £600-£900 estimate.
In one, she thanks Mr Dickman for his "thoughts and sympathy", following the death of the Queen Mother.
Several Christmas cards from the Queen, as well as from Princess Diana and the Prince of Wales, are also due to be auctioned.