The handwritten notes are part of a collection of Royal memorabilia from the late Mr Dickman's estate, which is to be sold through auction house Cheffins, and see Diana sharing intimate details of her sons' relationship throughout their childhood.
'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!...' one extract from a letter sent after Harry's birth in 1984 reveals.
'The reaction to one tiny person's birth has totally overwhelmed us and I can hardly breathe for the mass of flowers that are arriving here!'
Another letter sent in October 1992 reveals that Prince Harry has always been a cheeky chap.
'The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!" Diana wrote.
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.
Auctioneer Luke MacDonald told Sky News: 'They give a more personal insight into the lives of the royals (that) perhaps we can relate to far more than the more formal times we see them in the press and on television.'
'Her Royal Highness Princess Diana was adored by so many people. I think people feel that to actually have a letter, handwritten by her - it's something very special.'