The letters, which will be going under the hammer at Cheffins aution house on Januaray 5, show the correspondence between the People’s Princess and Buckingham Palace steward Cyril Dickman.
One such note, written just five days after the birth of Prince harry, shows the Princess thank Cyril for a note he sent welcoming the tiny Prince into the world.
Diana, then 23, wrote: "Dear Cyril, it was so kind of you to have sent us such a lovely card, on the arrival of our small son – we were both greatly touched by your thoughtfulness and enormously appreciated it.”
"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!"
She continued: "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! This is just to say a big thank you, Cyril. For thinking of us at this particularly happy time, with lots of love, from Diana."
In another heartwarming note, dated October 17, 1992, Diana reveals how mischievous Prince Harry really was during his time as a pupil at private Ludgrove School in Berkshire.
She wrote, “The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!”
It’s clear that the late Princess considered Cyril - who served as a palace steward for more than 50 years - as someone with whom she could be open with, once even sharing the shocking news of her grandmother’s death.
She wrote, “Thank you so much for your lovely letter concerning my Grandmother.”
“Her death came as a great shock, but it has meant an enormous amount to receive messages of support such as yours.”
“I often think of you and Mrs Dickman and hope that life is treating you kindly.”
In 2012, at the age of 85, the revered palace steward passed away and the letters were recently uncovered by his grandson who has since put them up for auction.
Speaking with the Cambridge News, son Matthew Dickman said, "He was much loved by the royal family. When he was ill, towards the end of his life, Prince Charles came to see him at his home in London."
With an estimated value of £15,000 ($AU25,000), the release of the letters has royal fans counting their pennies.