In one of the notes, written to her beautician on December 14, 1985 she writes: "I rushed home to open your Christmas present, which I thought was quite good considering the 25th is actually two weeks away!!
"I could not resist opening my present, as a parcel of any shape or form has never been safe with me and I fear that William has also picked up this dreadful habit from his mother."
Two years later, on Christmas day, she wrote to say thanks for the picture frame her beautician had given her.
"I can never get enough photograph frames for the amount of pictures I take of my small men and the arrival of yours is just perfect," she said.
"William and Harry had the best Christmas ever and I am exhausted just trying to keep up with them as they run around.
"Anyway at least they sleep well."
The 30 letters are expected to fetch in the region of £20,000 when they are auctioned in London on February 13.