Every time the poet Michael Rosen found out something new about what happened to his Jewish relatives during the Holocaust, he would send a round-robin email to his extended family. “My research brought me face to face with the destiny of Jews in Europe, seen through the prism of my own family.” Suddenly, he could imagine the “very real” journeys his relatives had taken, the places they’d hidden, the fear and hope they’d felt. “Because of the kind of person I am, I wanted to tell that story.”

The result is his latest book, The Missing, which will be published next week. A mixture of poetry and prose, it retraces Rosen’s journey as he searches for information about his European relatives who went “missing” before his birth in 1946.

When I confess to him that reading it brought me to tears, he explains how he coped with the pain of writing it: “Anybody who’s a writer knows you have that moment of the encounter, whatever it is. And then when you write about it, to make it safe for yourself, there’s a way in which you turn it into something else. It’s very hard to describe, but I know about it in relation to the death of my son. If you write about it, you put it outside of yourself.”

His son, Eddie, died 17 years ago of meningitis, at the age of 18. Rosen thinks the effort of pouring feelings into words can be transformative. “It gives you the chance to be objective. You can read it as if you are another – as if it’s not you who’s written it, it’s that bloke Michael Rosen.”

Rosen grew up in postwar Britain wondering about his missing relatives. He writes: “Stories hung in the air about French and Polish great-aunts and great-uncles, who were there before the war and weren’t after. ‘What happened?’ I’d ask. ‘Don’t know,’ my family said.”

They would have been full of hope for the future at the moment the Nazis arrived

The Nazis had two objectives, he points out: genocide of the Jews, and the destruction of Jewish culture and presence in Europe. The absence of any French or Polish cousins in his life had always been a painful reminder of this. “If, in our family, there was a ‘nothing’, then it felt as if that was almost a Nazi success. Even their names we were not 100% certain of, or where they lived.” The Nazis had created a gap where people in his family should have been, “and therefore they had won, not the war, but somehow over my family.”

And yet, he wondered, how could anyone – even the Nazis – successfully “disappear” a human being? There must be traces: letters, photographs, diaries, documents, some kind of paper trail to follow. “Frustratingly, for years, there wasn’t. And so it became this kind of challenge.”

There were two aspects of the book he says he found very difficult to write. The first was his discovery, after some amazing detective work, that his paternal great-uncle Oscar and great-aunt Rachel had almost made it out of France in 1943. They missed the boat they planned to escape on by just a few days and were then captured in Nice – in fact, “it could have been a matter of hours,” Rosen says. “It was all so near, and so far.”

They would have been full of hope for the future at the moment the Nazis arrived. But instead of sailing to freedom, they were immediately deported to Auschwitz on a cattle-truck train. A moving poem in the book, Dear Oscar, explores what they must have gone through on that journey: “Did you see in the dark / horror on Rachel’s face? / Did she see horror on yours?”

Poetry like this appears within the book at key moments. Writing poems, Rosen says, helped him to reflect on what he was discovering and, most importantly, it allowed him to search for the feelings and memories that are missing from the factual documents he finds. “It opens up forms of empathy and emotion.”

The second piece of research Rosen found hard to write about was the report of his other paternal great-uncle Martin’s arrest. Four French police officers, acting on instructions from the Nazis, describe how they entered their fellow Frenchman’s house at 2.30am one wintry night and arrested him because, as they note down, he is a Jew. In a handwritten document, they state his appearance in officious detail: 162cm tall, brown eyes, oval face, straight nose. A scar on his left cheek. They even note down his meagre clothing: “Yellow cotton trousers, a grey cotton jacket, a Basque beret and low-heeled shoes.”

It was reading this description in the officer’s own hand that distressed Rosen the most: “The humanness of it – that I found very difficult. The fact that they’ve described what he’s wearing, his height, even his shoes... it’s the banality of evil that [philosopher and political theorist] Hannah Arendt talked about.” He thought about the officers standing over his uncle, jotting all these details down for a report they were going to write up afterwards. “And then it’s, sort of, job done.”

Martin was handed over to the Nazis and, like his brother, sent to Auschwitz. Rosen thinks he would have been taken by surprise – the order to arrest Jews had gone out only a few days earlier and his uncle wouldn’t have known about it. “Seeing the vulnerability, the speed with which he was arrested… it went into [my] ‘vulnerability river’.” He explains that ever since his son died he has been feeling more vulnerable in general and that writing the book fed this particular river of emotion.

“I do think about vulnerability differently since then.” Eddie was his second son. Rosen, 73, who brings his third wife, Emma, to our interview, is now the father of five children and two stepchildren. He says that when he sees one of them fall over or get a cough, he becomes painfully aware of what they have been through and how vulnerable they feel. “I have to do a little check inside myself… And then I have to make a little check back that all these people are still living, aren’t they, and to remind myself that it’s not that everybody’s dead.”

My family’s life is utterly European. I’d never fully sensed that before

His mother used to tell him that if the Nazis had invaded England, he’d never have been born. Finding out what happened to his relatives demonstrated the truth of those words, he says, and made him realise that they were not just victims; they were also refugees.

It has made him feel more European. “If you study history, as I have done, the way in which the narrative of Britain is told is that it’s separate from Europe. But I guess my life and my family’s life are utterly European. And I don’t think I’d fully sensed that before.”

He sees shadows of the way his family were treated by the Nazis in the way refugees and asylum seekers today are treated here and by Donald Trump. “Rounding up people, putting them into containments, imprisoning them, taking their papers away – all that is having resonance with me.”

Unusually, the book is aimed at children aged 10 and over, as well as adults. Rosen decided to write it that way after visiting a school where a pupil denied the Holocaust to his face. “This young man put up his hand and said: ‘It didn’t happen, did it?’” As the teacher panicked, Rosen remembers counting to three and patiently saying: “Well, no, it did happen.”

Until then, Rosen (a former Children’s Laureate and the son of two teachers) hadn’t planned to write a book about his family at all. But after going into other schools to talk about the Holocaust, he realised he could explain what had happened to his relatives in a way that children could understand and engage with. “I thought: this is a meaningful thing to do, this is valuable.” Plus, he suspected children would be interested in reading about the mystery of his missing relatives – and how, by following “these tiny, separate bits of paper” he had managed to find out what had happened to them. “I do see myself as a storyteller and who do I tell stories to? Largely young people.”

It is important, he says, that people – particularly children – understand “how these things happen in such banal and ordinary ways. It’s the contrast between the ordinariness of my life, and how near this stuff is to it.”

Perhaps, Rosen says, his book will inspire other people to tell their own stories. Perhaps one day, his own descendants will read it. And while nothing will ever fill the gap the Nazis created when they murdered Oscar, Rachel and Martin, writing the book has brought him closer to family members all over the world. “I’ve met with first and second cousins who were related to them. I’ve put them back into the family. And that’s very powerful.”

The Missing is published on 2 January by Walker Books at £8.99; order from guardianbookshop.com for £7.91. An exclusive extract follows

The Missing by Michael Rosen: an extract

“What happened to them all – the brothers and sisters in Poland and France?” I asked. Dad shrugged.

“I don’t know,” he said. “They were there at the beginning of the war, but they had gone by the end. I suppose they died in the camps.”

In the camps? I thought. What camps?

It made me think of school holidays: we used to go camping and it nearly always rained…

But Dad didn’t mean camps like that.

Sometimes, on TV, they showed what concentration camps were like. They said that hundreds of thousands, probably millions, of people were killed in these camps.

I used to lie in bed and think about this. It seemed horrifying. Awful. But I didn’t really get it.

Why did they put my great-aunts and great-uncles in these camps? And why did they kill them?

And now they were gone. No way of finding out anything at all about them. They had just disappeared. We didn’t even have any pictures of them.

All that was left was my dad’s shrug.

The Absentees

There are gaps,

there are blanks,

in the house of my life;

there’s a face,

nothing more,

something gone from my life.

She was here,

he was there,

in the rooms of my life;

there’s a place

for them both in the words

of my life.

Written for Holocaust Memorial Day, 2018