Princess Diana鈥檚 letters to her family鈥檚 former housekeeper are going on sale, providing an 鈥渋nsight鈥 into her life behind closed doors, including wedding preparations to the then-Prince Charles, and details of Prince Harry and Prince William鈥檚 childhood relationship.

Violet Collison, nicknamed Collie, was the head housekeeper at Park House on the royal residence of Sandringham Estate in Norfolk, where Diana was born and raised. Collison witnessed the birth of Diana as well as her siblings, Sarah, Jane and Charles, according to Sworders, the auction house handling the sale on July 30.

鈥淸Collison] was a constant in Diana鈥檚 life, and remained so all her life,鈥 Luke Macdonald, head of art and estates at Sworders, told CNN Thursday. 鈥淔or Diana, she was always there.鈥

Even after Collison retired in 1973 Diana would pay visits to her former housekeeper, according to Macdonald, who added that Diana would 鈥渟lip away鈥 for a cup of tea in Collison鈥檚 Norfolk home when she was at Sandringham.

鈥淭hey were very close,鈥 said Macdonald, adding that the two would correspond for birthdays, Christmases, and even before Diana鈥檚 wedding to then-Prince Charles.

One letter dated July 8, 1981 鈥 three weeks before the royal wedding 鈥 was handwritten by Diana on Buckingham Palace stationary, telling Collison about the pre-wedding preparations.

鈥淓veryone frantically busy here doing last minute decorations鈥 the bride to be has remained quite calm,鈥 Diana wrote.

Another letter, written 10 days after the birth of Prince Harry in 1984, thanks Collison for a gift she sent the newborn.

鈥淚t was so very kind of you to have given such a lovely present to Harry 鈥 we were enormously touched,鈥 wrote Diana.

In the same letter, Diana tells Collison about Prince William鈥檚 adoration for his little brother. 鈥淲illiam adores his little brother and spends the entire time pouring an endless supply of hugs and kisses over Harry,鈥 she wrote. 鈥淲hich is wonderful to watch, if we鈥檙e allowed near.鈥

鈥淭he letters are an insight into Diana鈥檚 life under the radar, sort of beyond the royal protocol,鈥 said Macdonald, who thinks 鈥渢hey only really re-confirm how gentle and loving and kind and nice she was.鈥

That same year, Diana sent Collison another thank you note for sending Christmas presents to the boys.

鈥淗ow wonderfully kind it was of you to give our small sons such marvellous presents for Christmas,鈥 wrote Diana on personalized Kensington Palace stationary, adding that 鈥淲illiam found the parcels and dived into the paper before I could stop him.鈥

Among the items up for auction are also signed family Christmas cards and memorabilia from events relating to Diana鈥檚 life, such as a personal invitation to the royal wedding, a ticket for Diana鈥檚 funeral and correspondence for her memorial service.

While some of the letters are listed with an estimated sale price of £800 to £1,200 (US$1,030 to US$1,545), Macdonald imagines the interest they have generated means they could sell for more. 鈥淚t wouldn鈥檛 surprise me if one or two of them may double or more,鈥 he said.

鈥淭hey are one person鈥檚 whole story from start to finish, all in one group of letters and Christmas cards鈥 They really are a chapter from Diana鈥檚 childhood right through her life.鈥