Joe Steiner hurries into the lobby of his apartment building in an unfashionable part of midtown Manhattan. Every day he reverse commutes to New Jersey, where he is a salesman for a wholesale stationery company, and he has been kept late at the office. Apologising, he leads me to a nearby bar and grill for a beer and an early supper. We choose a table as far as we can from the TV in the corner, where a cluster of men are watching a basketball game.
The mundane backdrop heightens the impact of Mr Steiner's extraordinary story. More than half a century ago, he was one of the few survivors of the Warsaw ghetto. He and his family hid in a rag warehouse there for more than two years until, as a nine-year-old, he escaped with an elder sister only hours before the German SS razed the ghetto in 1943. His mother and another sister had already been sent to a concentration camp, and he was never to see them again.
Like all the people I interviewed for Witnesses to War, Mr Steiner described these dreadful experiences and his feelings about them with little emotion. As my research progressed, I began to understand why.
They had all grown up in 1930s Europe, where fear and uncertainty were part of the currency of life. They had never known it otherwise. Although their parents tried to shield them from the worst horrors, they could not fail to detect the menace, the sense that the comforting routines of their lives could be ruptured at a moment's notice. They developed a survival mentality because there was no alternative.
Two revealing moments in my interviews brought the point home. The first was in Newcastle-upon-Tyne, where I met Alexander Michelowski, a Catholic priest ministering to the Polish community. At 11 he had been snatched from his Polish house by the SS and sent to an institution where he was brought up as a German. What he remembered most vividly were not the freezing cold or the painful forced marches through the countryside, but the daring, illicit midnight feasts with the other children.
Then I went to Paris to see Alice, who escaped with her mother and brother from occupied France into Switzerland, climbing through a barbed wire fence after a long, perilous walk by torchlight. Half a century later, in her comfortable apartment on the Boulevard Raspail, she recalled that her main emotion had not been fear but excitement. At 12, she was a character in a real-life adventure. While she knew it was not precisely a game, it was certainly a thrill.
That gave me the key to how to put these dreadful events into words. I had never written for children before, and people ask whether it was hard to adjust my style. The answer is no, because the men and women I interviewed were taking themselves back in time and recreating their childhood perceptions. If I relayed what they said more or less straightforwardly, it would make sense to boys and girls of the same age. Without downplaying the horror, I could balance it with the sense of adventure that, to them, was just as important a part of their experiences.
The idea for a book about the wartime suffering of European children came from Rosemary Stones when she was associate publisher at Penguin Children's Books. She contacted me after she had read Master Race, a book about the Nazis' racial philosophy that I co-authored. Alexander Michelowski's story comes from that book.
I was surprised how easy it was to trace other survivors. Suddenly I seemed to stumble across them everywhere. I first met Bea Green at a friend's house, without knowing that she had come to England on one of the Kindertransport trains from Germany in 1939; so I was amazed when I saw her mentioned in a book on the subject. Then I met a former colleague in the London Library and told him what I was working on. "You should talk to my ex-wife," he said - and that was how I found Alice.
I read a book called The Hidden Children, whose author, Jane Marks, had interviewed dozens of Americans who escaped from Europe when young, at a convention organised for them in New York. Now, as they entered the final phase of their lives, they were ready to talk, often for the first time, about experiences that many had until then buried deep in their consciousness. Ms Marks put me in touch with two of my subjects, including Joe Steiner.
The most disturbing part of the research was my visit to Lidice, the Czech village razed by the Nazis in a vicious reprisal raid, its residents shot or dispersed. After the war the Government could trace only a few of the children, but built a new village for them on the site of the old one and turned it into a permanent monument to Nazi inhumanity, constantly visited by tourists and VIPs.
The survivors are still there, telling and retelling their stories, trapped in their past. Unlike Joe Steiner and the others, they have not been allowed to move on, to affirm the human resilience and fortitude that Witnesses to War finally celebrates.