Agatha Christie has come to be seen as the epitome of cosy crime. With the dandified Poirot or the seemingly innocent Miss Marple at the helm, her stories have a veneer of bucolic English life, temporarily derailed by nefarious deeds. But Christie was never cosy. Her novels contain hidden, darker depths with characters, plots and incidents dredged from her own troubled existence.

Author Cathy Rentzenbrink is a Christie fanatic, who found herself in the midst of a series of crises, both personal and professional, and turned to the Queen of Crime for comfort. But as she reveals in her new book , the innumerable problems that Christie faced often reflected her own.

“As my own life derailed,” Rentzenbrink writes. “I felt she had something to teach me about disappointment and heartbreak.”

Rentzenbrink read all of Christie’s books in order, witnessing the author’s life unravel before her, especially when she reached 1926. That was the year that cemented Agatha’s reputation, but also the year that nearly destroyed her.

Though considered a quintessentially British writer, Agatha Christie was always an outsider, and utilised the perspective of an interloper spying on the bizarre activities of the people around her. She was born Agatha Miller – the youngest of three children – into a wealthy upper middle-class family in Torquay, Devon. Her father, Fred Miller, was a wealthy, gregarious American, while her mother, Clara, was born in Dublin with a half-German mother. When Agatha was born on 15 September, 1890, the family were rich, influential members of Torquay society. But Fred has his secrets.

His fortune soon dwindled, a fact he kept from the family. When he died suddenly, in 1901, they were plunged into financial uncertainty. Christie’s stable world crumbled in an instant. Her mother, completely undone by grief, appeared to transform into a different person. Her later work was filled with characters who are never exactly as they seem, just as her father and mother appeared to alter before her.

While Christie dreamed of higher education, her mother’s plan was to find her a rich husband and secure the family’s finances. In October 1912, at a local dance, Agatha met Archie Christie, a handsome, dashing pilot. They fell in love and quickly married, only to be almost instantly separated.

At the outbreak of the First World War, Archie was sent to France to fight. Christie volunteered as a nurse at a Torquay hospital, treating injured men returning from the front. This would have a profound effect on her as a burgeoning writer. For the first time, she witnessed people from all social classes intermingling. She also saw how completely incompetent those in charge could be. It left her with a lifelong distrust of authority figures. Her later books were filled with those seemingly in charge who singularly fail in their duties.

Her post also involved her assisting the local pharmacist, which gave her an in-depth knowledge of poisons, something that would prove invaluable in her plots. She soon realised that with the right information, the perfect murder could be quite easy to achieve.

She bundled all of this new-found experience into The Mysterious Affair at Styles, her first foray into detective fiction written in 1916. The setting was a large but shabby country house, much like the one she grew up in. The victim was poisoned with strychnine, with which Christie would have been familiar with from her medical duties. And the plot is populated with rather useless, vain, over-privileged men, as she witnessed during her time in the hospital. And as Rentzenbrink points out, it’s also a book full of anguished love. “How astonishing to think she wrote it while Archie was in France and the outcome of the war was still in doubt,” she writes.

For her detective, Christie created a character hewn from the people she saw around her. Devon was filled with Belgian refugees fleeing the horrors of war. Christie knew, as an outsider herself, that a figure like this would have the perfect perspective on the British psyche. Hercule Poirot was dapper, calm and meticulous to a fault. It was a completely new type of detective, but he immediately captured the public’s imagination.

The book was well received and more followed. She was now establishing herself as a crime writer of merit, while acting as a devoted wife and mother to her daughter Rosalind. But all this came crashing down in 1926.

The Murder of Roger Ackroyd was her sixth novel and was published to acclaim as the year began. But as it sat at the top of the bestseller’s lists, a series of events nearly destroyed Christie. Just as she was starting to feel the pressure of her relentless publishing schedule, her beloved mother Clara died. Grief-stricken, she found she could no longer write. Then in the midst of this crisis, Archie demanded a divorce. He was in love with a friend of Christie’s and they wished to marry at once.

A distraught Christie dissolved. Her car was discovered teetering on the edge of a chalk quarry, with her coat and expired driving licence inside. She was nowhere to be found. A huge manhunt plus breathless tabloid press coverage followed, the mysterious disappearance of a mystery writer being too good to ignore. She was eventually discovered in a hotel in Harrogate.

It was claimed the car accident had left her with memory loss. But no one bought the story. To this day, what really happened in Harrogate is unknown and she rarely spoke about it. But the coverage turned her into a household name and would have a profound impact on her work.

Her thrillers started to adopt a more psychological aspect. Though it was never confirmed, it was assumed that Christie engaged in some sort of therapy or analysis as she recovered from her apparent breakdown. Soon Poirot was claiming that material evidence held less interest for him and it was the criminal’s mental activity that proved more useful.

She also introduced a new detective. Created, as Rentzenbrink describes her, “as a defence mechanism, a refuge and a secret revenge”. Betrayed by Archie, pursued by the men of the press, Christie found solace in a little old lady with tremendous fortitude: Miss Marple. Unlike the besieged Christie, Marple, as an elderly woman, was overlooked and underestimated. But, in detection, this was her superpower. The Murder at the Vicarage, the first Marple, was published in 1930.

Now divorced and with her daughter in boarding school, the recovering Christie found herself free to both write and explore. She began a life of adventure, traveling across the Middle East to visit historic sites and eventually meeting and marrying Max Mallowan, an archaeologist 14 years her junior, who would remain by her side for the rest of her life.

With Max beside her, Christie would go on to have her greatest successes, including And Then There Were None, Murder on the Orient Express and Death on the Nile. By the time of her death in 1976, she was the most successful novelist (and playwright, thanks to ) on the planet. Only Shakespeare and the Bible have outsold her. To the end, her work continued to contain the harsh realities that she’d suffered in her life, exorcised through fiction.

“We think of her as the Queen of Crime,” Rentzenbrink writes, “but she is also the little girl – 11 years old – whose jolly father died, taking the laughter and security with him.”

‘The Agatha Christie Cure’ by Cathy Rentzenbrink is published on 10 September by Canongate