A collection of never-before-seen photographs and letters that give an insight into Agatha Christie's private life have emerged 45 years after her death. The photographs show the legendary crime writer celebrating her 81st birthday with her second husband, Max Mallowan, and unnamed friends on Dartmoor, Devon, in 1971, five years before her death in 1976.

The 26 autographed letters and cards were written to the author's long-time friend, Elizabeth Callow. A descendant of Ms Callow has now decided to put the archive up for sale with Chiswick Auctions of West London, and it is expected to fetch $4,800 (£3,500). Pictured: Agatha (left) next to her husband, Max Mallowan, and friends. 

The letters cover much of Christie's later life when her health started to fail. In the letters, Christie talks of the 1971 birthday occasion as well about a snowy Christmas at her Devon property, Greenway, and other snippets from her private life.

She does mention her work occasionally, with one letter from October 1970 referring to a shortage of copies of her spy novel 'Passenger to Frankfurt'. Christie wrote: 'I'd send you Passenger to Frankfurt but I've run out of it – owing to the birthday rush – and Collins are having to bring out a new edition already'. Pictured: Christie posing for her birthday snap while celebrating her birthday with friends in Dartmoor.

In early January 1971, she told Miss Callow of the family Christmas she had just enjoyed in Devon. Upon returning to her main home, Winterbrook House in Wallingford, Oxfordshire, she wrote: 'We have just got back from a nice but hectic and snowy Xmas at Greenway. Very cold and frosty outside. Practically everyone had colds including the children, only I escaped. Perhaps germs don't like the elderly.'

She mentioned her 81st birthday in a letter written in September 1971. She said: 'I had an agreeable birthday in Devon – a picnic, lovely mild weather on Dartmoor.' The same letter also highlights her witty sense of humor. She wrote: 'I hope I've got the right Elizabeth?!! Nowadays when one only relies on one's friends' Christian names – and as one usually communicates by telephone so that you don't really recognize handwriting – difficulties do arise.' Pictured: Christie taking a break on a bench during her grand day out on Dartmoor.

Her health took a turn for the worse later in 1971. In another letter from October 1971 she wrote about breaking her hip after tripping on a rug. She wrote: 'I, as you may have heard, fractured my hip in early July, it had to be operated on... But I get along on sticks quite well. But what an idiotic thing to do – tripped over the corner of a rug.' She mentions it again in May the following year when she said: 'I was also rather lame and had to use sticks again. My surgeon said my hip was perfectly all right - this was what he called an aftermath. Don't bend too much in your market garden and put a hot water bottle to your back at night!' Pictured: The writer's friends and husband (left) in the birthday snaps.

Christie sold an estimated 300 million books in her lifetime and is the best-selling fiction writer of all time. Her last novel, Postern of Fate, was written in 1973. She suffered a heart attack and another serious fall in 1974, after which she was unable to write. Pictured: Christie enjoying a lazy picnic lunch under a tree on Dartmoor with her husband (second from right) and friends. One friend lies flat on his back, while her female companions are dressed appropriately in light summer dresses.

She died aged 85 in 1976. Also included in the archive are 14 letters and cards from Christie's archaeologist husband Mallowan, mostly covering work-related matters – as Elizabeth Callow was also an archaeologist – but including a letter written after Agatha's death, thanking Elizabeth for her kind words. Valentina Borghi, from Chiswick Auctions, said: 'The letters are an account of her life and what she was doing, they don't really go into her work but provide us with an insight into her private life.' Pictured: Christie's second husband, Max Mallowan, holding a bottle of champagne.

Borghi continued: 'They have never been seen before, they have been kept by Elizabeth Callow's family, tucked away in the attic for years. They want them to go to someone that can enjoy them more. Elizabeth Callow worked in archaeology and did work on some research projects alongside Agatha's husband Max Mallowan, that's how they met.' She added: 'It was a very close friendship, the letters cover about 20 years, and she was also close to Max, writing to him when Agatha died.' Pictured: Christie resting with her husband (standing, third from left) friends and their pets beside a cluster of rocks on Dartmoor.

Borghi also shared: 'As a collection, we think this could do really well. Agatha Christie is a very strong name on the market and autographs by her are not that common. Collectors really have to wait to get something from her.' She continued: 'With the Death on the Nile film that was due to be released last year but delayed due to Covid, I think there will be a renewed interest in her work and her life.' Pictured: The smartly dressed group enjoys a picnic with bottles of beer seen arranged on the grass. The dogs had their leads slung over a post to stop them running away.

The archive will be sold in London on November 9 at Chiswick Auctions in West London.

Christie had an illustrious career as a wordsmith, writing a total of 66 detective novels, a series of short stories and the world's longest running play, The Mousetrap. The most intriguing aspect of her life, however, is her own disappearance from her Berkshire home in 1926. Historians have been debating for nearly 100 years as to exactly why she vanished during the height of her fame, leaving her home in Sunningdale after kissing her 7-year-old daughter Rosalind goodbye. She was found 11 days later after a search involving a thousand police officers, tracked down to a hotel in Harrogate, North Yorkshire, and claimed she couldn't remember a thing. Pictured: Agatha Christie and her husband, British archaeologist Max Mallowan, leave their home in London in January 1933.

At the time of her disappearance Christie, who was 36, and famous for her Miss Marple and Poirot detective novels, was grieving for her mother. Also her husband, Colonel Archie Christie, a pilot in WWI, had just announced he wanted a divorce because he was in love with a younger woman. A huge manhunt was launched and her car was found abandoned between Dorking and Guildford in Surrey. Even Sir Arthur Conan Doyle, author of Sherlock Holmes, joined in with the search that made national newspaper headlines. Some claimed she had drowned in a nearby pool, but her body was nowhere to be found. She was finally found when a musician at a hotel in Harrogate called the police when she checked in with no luggage and used the name Teresa Neele - the name of the woman her husband was in love with. Christie went on to divorce Archie in 1928, and married Max Mallowan in 1930. She was made a Dame in 1971, and she died in January 1976 at the age of 85.