In what ways could she be seen as a feminist?

Throughout her life, Agatha kept up a public image that she wasn’t a professional working writer – which, clearly, she was. She wrote roughly 80 books; 2 billion paper copies of her works have existed; she’s sold more copies than any other book or writer except the Bible and Shakespeare. What strikes me is that, unlike God or Shakespeare, she’s female. And that often gets weirdly glossed over.

That’s partly her own doing. She never wanted to be taken seriously as a professional writer, partly because of the values of her childhood but also because of something very bad that happened to her in 1926. That year marked the great pivot point of Agatha’s life, when she went through a huge episode of what we’d today call public shaming, which we’ll return to later.

At the time, lots of newspaper articles were written about her in which her professional success was definitely a bad thing. So, after 1926, she never spoke of herself in those terms again. You’d think that on her passport she’d state her profession as writer or author – but it actually said she was a housewife.

You say that, in terms of her parents’ expectations for her life, getting married was the biggest priority. So did her first marriage conform to social norms of the time?

She came on to the marriage market in trying circumstances: her father had died and the family had lost their money, so they were going downhill socially. Yet Agatha was still expected to try to marry up. She actually received nine proposals, sometimes dismissing them in the funniest ways. When one young man asked her to marry him, she said something like: “It’s an awfully silly thing to ask a girl to marry like this. We’ve only known each other for 10 days – don’t be such a fool.”

But then along came the 10th man, Archibald Christie – Archie. He wasn’t quite what her mother would have wanted, being from a slightly lower social class. In his favour, though, he was incredibly hot, and a pilot, and Agatha was head over heels with him. It’s like the film Top Gun, except it happened in a country house in Devon in 1912.

There was a lot of toing and froing over whether they would get married, because neither of the families was particularly happy about what they saw as this slightly ill-matched couple. Perhaps they wouldn’t have tied the knot had it not been for the First World War. Archie was sent off to France, and when he came home for Christmas after the first few months of the war, something had changed within him, and he said: “We’ve got to get married at once.” It was a very hasty wartime marriage, like many others. And, like many other hasty wartime marriages, it wasn’t going to last that long.

What was Agatha’s own experience of the First World War?

I don’t think it’s possible to overestimate just how important the First World War was in the creation of Agatha Christie as a person. With Archie away serving in France, she volunteered at the field hospital that had been set up in Torquay. She started at the bottom, washing the floors, and she later used this experience in her fiction; you find little snatches of her real life in so many of her novels. Then she progressed to actually treating patients, which was harrowing work. She describes having to take a man’s amputated leg down to the furnace in the hospital for it to be burned, and how one of her patients died after three days in her personal care.

The trauma of war clearly affected men – everybody knows about shell-shock – but historians have realised recently that it affected women, too, in a way that nobody really wanted to talk about at the time. Even if the nurses weren’t at the front line, they still experienced things that no nice young ladies like Agatha ever expected to witness – the naked, dirty and wounded bodies of young men, for instance. Then they had to go home to their families, but they could not distress them by describing what they’d seen. They had to keep this horror within the hospital. This act of containment – having a secret, being not quite what you seem from the outside – was essential to all of the characters Agatha went on to produce as a detective novelist.

Her time in the hospital was also particularly important because she trained as a pharmacist’s assistant, taking a job in the part of the hospital where the medicines were mixed – this involved handling really dangerous, strong drugs. So Agatha was handling poisons, and it was during her time working in the pharmacy that she picked up a pen and started to write. One of the things she wrote, beginning in 1916, was her very first published detective novel, The Mysterious Affair at Styles [which originally appeared in 1920]. And guess what?

It features a young lady who works in a hospital as a dispenser, and it features a death by poison.

• Read more | Arsenic: a brief history of Agatha Christie’s favourite murder weapon

Let’s talk about 1926 and Agatha’s 11-day disappearance, an episode that captured the nation’s imagination. What was the background to her vanishing?

This mysterious disappearance of Agatha Christie, which led to a national search for her – she was eventually found staying under a false name in a hotel in Harrogate – seems to be the central injustice of Agatha Christie’s life. A lot of people would tell you that it happened because she wanted to frame her cheating husband for her murder. That’s what the newspapers said at the time, and it’s a narrative that’s been picked up in so many other books about her.
When I started my research for this book, I was open to that suggestion. But it’s not at all consistent with what I found in her personal archive and in the evidence from the time. What I believe happened was as different from that as night from day.

By 1926, Agatha was a successful novelist, and she was under a lot of pressure to keep producing books. But her mother died that year, and she went into an episode of what today would probably be described as depression. She reported forgetfulness, tearfulness, insomnia, an inability to cope with normal life. Unfortunately, it turned out that her husband, Archie, couldn’t cope with this – he wasn’t an emotionally literate man. And then he announced that he was leaving her for a woman called Nancy, who was nearly a decade younger than Agatha.