'An insight into Diana's life': Princess' handwritten letters go on sale

Princess Diana's letters to her family's former housekeeper are going on sale, providing an "insight" into her life behind closed doors, including wedding preparations to the then-Prince Charles, and details of Prince Harry and Prince William's 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.

"(Collison) was a constant in Diana's life, and remained so all her life," Luke Macdonald, head of art and estates at Sworders, told CNN Thursday. "For Diana, she was always there."

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

"They were very close," said Macdonald, adding that the two would correspond for birthdays, Christmases, and even before Diana's wedding to then-Prince Charles.

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

"Everyone frantically busy here doing last minute decorations... the bride to be has remained quite calm," Diana wrote.

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

"It was so very kind of you to have given such a lovely present to Harry - we were enormously touched," wrote Diana.

In the same letter, Diana tells Collison about Prince William's adoration for his little brother. "William adores his little brother and spends the entire time pouring an endless supply of hugs and kisses over Harry," she wrote. "Which is wonderful to watch, if we're allowed near."

"The letters are an insight into Diana's life under the radar, sort of beyond the royal protocol," said Macdonald, who thinks "they only really re-confirm how gentle and loving and kind and nice she was."

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

"How wonderfully kind it was of you to give our small sons such marvelous presents for Christmas," wrote Diana on personalized Kensington Palace stationary, adding that "William found the parcels and dived into the paper before I could stop him."

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

While some of the letters are listed with an estimated sale price of 800 to 1,200 ($1,030 to $1,545), Macdonald imagines the interest they have generated means they could sell for more. "It wouldn't surprise me if one or two of them may double or more," he said.

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