Liz Kessler has written about mermaids, a pirate dog, time travel, spectral romance and teenagers coming out as gay, but her latest book, When the World Was Ours, is, she says “the one that’s been in my heart a long time”.

“A couple of years ago I’d got to the point where I’d been writing for nearly 20 years and I wanted to jump off the hamster wheel, in a way,” she says. It was time to write the book she had been thinking about for so long. She especially wanted her father, Harry, who is 90, to be around to read it — because it is inspired by a life-changing incident in his childhood.

It began on a Danube steamer, before the Second World War.

“My dad was four years old and he was kneeling on a seat. My grandfather said, ‘be careful, you’re going to scuff that woman’s dress’.” The couple — William and Gladys Jones — who were in Vienna for a dental conference, began to chat to Kessler’s family and missed the stop where the rest of their touring party had alighted. So they went home with the Kesslers instead, enjoying Liz’s grandmother’s home-made Sachertorte and, the following day, being shown around Vienna. When the Joneses returned to England, they sent a thank you letter. That letter would save the Kesslers’ lives.

In 1938 when the Anschluss brought the Nazi threat close to home, the Kesslers fled to Czechoslovakia, but to get to safety in England they needed a British family to vouch for them. Harry’s father found the letter, wrote to the Joneses, who immediately agreed to help — and so the Kesslers were rescued.

“My dad and his parents went to live with the Joneses,” says Kessler, marvelling at “the fact that this tiny moment changed everything.”

When the World Was Ours follows the lives of Jewish Elsa (named after Kessler’s great-aunt, who was murdered in Auschwitz), non-Jewish Max – and Leo. Leo is Harry’s fictional counterpart but Kessler insists Leo is “his own character” and in the book the meeting with “Mr and Mrs Stewart” happens on a Ferris wheel rather than a steamer.

“I relocated the incident because, when we were in Vienna, I was hoping to see a Danube steamer of some sort and get the feel for how it might have been,” says Kessler. “The boats that run nowadays are much more modern and I wasn’t sure if I’d be able to get the information I wanted. In the meantime, we decided to have an evening at the Prater fairground to cheer ourselves up after all the difficult stuff we were doing. Whilst there, we went on the Riesenrad Ferris wheel and I had such a strong sense of its history — as well as the fact that it felt like a good idea for a birthday treat for a young boy — that I realised it was the perfect setting for this part of my story.

“I always feel the need to go to the place that feels right for my books, even if they are based in fictional settings,” she says. For the mermaid series, she went to Bermuda and on a Norwegian cruise, among other journeys.

“But none of them had been quite like the research trip I did last year.”

As well as a year intensively reading about the Holocaust, Kessler and her wife Laura toured five countries in a campervan, visiting four concentration camps.

“This was the hardest thing I’ve ever done,” says Kessler, and she had various friends, family members and Rabbi Robyn Ashworth-Steen standing by at home, to call upon for extra support when it became unbearable.

“Each camp affected me differently. After Dachau, I was heartbroken; after Mauthausen, enraged. The weather was beautiful as we drove up [to the camp]. At the end was this scene of mass murder — and beside it were fields where the SS would invite local people to play football at the weekend.

“At Auschwitz-Birkenau I was distraught at the time. Afterwards I thought I was just about OK, but over the few days that followed it felt like Auschwitz had crawled under my skin. My mental health started to go quite ragged so we went to Amsterdam quite quickly. I’ve given this book everything I’ve got and plumbed the depth of my emotional resources and my family’s history.”

When the book opens in 1936, Elsa, Leo and Max are firm friends, but Max slowly becomes converted to Nazi ideology. Kessler delicately and painfully anatomises his psychological struggles, akin to being drawn into a gang of bullies.

“In a way I think Max’s story was cathartic,” she says. It was her way of exploring the question ‘How might someone find themselves enrolled in something so terrible?’

“I’m sure every Jewish person, every human being with decency, must have thought… how did an entire nation take part or turn away?’”

To write such a book for readers age 12-plus involved careful decision-making. “In the past, I’d think: ‘What do my readers want? Do they want Emily to kiss a boy?’ for example.” But this time, truth led the way.

“I just think there’s no point doing the book unless I’m true. I’m not writing a fairy tale,” she says. And yet there was a balance to be struck. “I sat on a bench outside Auschwitz and said to Laura, ‘I don’t want young people to feel as broken as I feel now.’ It felt extremely important that I didn’t fudge the facts. But also that it was a fictional story about hope and friendship,” — and in that sense it was no different from her other books.

“I didn’t even know if it would be a book for young people,” she says. “We agreed that it would be aimed at an audience of 12 plus. It definitely isn’t a book to go into primary schools — and I hope it will appeal to adults too.” Kessler is working on an idea for her next book, but says “going back to younger fantasy stuff doesn’t feel right at the moment”. She and Laura live on a canal boat with their “crazy” puppy Lowen — “a (gorgeous) working cocker spaniel”. They have moved from Cornwall to the north west to be nearer to their families — including Harry, of course. “Dad is very excited and it’s lovely to be sharing the book with him.”

When the World Was Ours is published by Simon & Schuster