It wasn’t until Margreeth van Vliet-Smit was diagnosed with inoperable cancer in 2001 that Elma van Vliet realised how much she didn’t know about her own mother. She didn’t know what dreams and adventures Margreeth had had when she was a young woman. She didn’t know what her mother had done on Saturday afternoons as a small girl. She didn’t even know how to make the chicken soup Margreeth had always made when her daughter was ill. “And she was the only person in the world who knew how to make that soup,” said Elma.

So Elma began jotting down questions for her mother, to tease out who she was – and why. “As soon as I started writing, the questions kept coming. I realised that when we had talked, it had always been about my life, not hers,” she said. Elma’s questions ranged widely, from “What kind of parents did you have: were they progressive or old-fashioned?” to “Were you an easy-going or rebellious teenager?” and “What changed in you when you had children?”

As the questions flowed, themes began to emerge, ranging across childhood, teenage years, hobbies and having children. The themes morphed into chapters, and it soon became obvious to Elma that the chapters had become a book.

Daughters often only think to ask after they have lost their mum

Not everyone was immediately convinced. At least one friend told her that nobody would buy an empty book. “But I knew to ignore them because if they were looking at it as an empty book, they didn’t understand,” she said. As Elma suspected, the book’s naysayers were proved wrong. First published in 2004 – Margreeth was given the very first copy – sales of Mum, Tell Me began to soar in 2016 after newly designed editions were released.

It has now sold more than 1m copies in the Netherlands, Denmark and Germany, sparking a series: Tell Once, comprising books for fathers, grandmothers and grandfathers. This second swathe has sold another 2m copies. In Denmark, one in 10 households owns at least one of the titles. According to her German publisher, Elma van Vliet is now the “most internationally successful Dutch author ever”. Mum, Tell Me was published in the UK earlier this month. The grandmother book will be on the shelves in October and the publishers hope the father’s book will follow soon after.

“When I started writing Mum, Tell Me, my only intention was to make a book for my mother because I had so many questions I had never asked her,” said Elma. “I never had the intention for it to become such an incredible journey.” The journey has been incredible not just for Elma but for the three million people in 12 countries who have now written down their stories for their children and grandchildren who can now hold their own, deeply personal family history in their hands.

What is different about the Tell Once series is that it’s about finding yourself by finding out where you come from. As well as creating a handwritten family history, the questions can trigger conversations that daughters often only think to ask when they’ve lost their mothers – or when they’ve had their own children and begin to appreciate what being a mother means. Questions about how their mother coped with childbirth, the work-life balance, parental guilt and the teenage years – what they’d learned and what they’d do differently if they could do it all over again.

Facebook Twitter Pinterest ‘We talked on the sofa for the first time – sitting there for an hour, just chatting’: Elma van Vliet. Photograph: Sarah Wong/The Observer

The book, said Elma, is about learning to love yourself by learning to understand – and thereby love – those who created you. “Mothers and daughters can often be judgmental about each other. I was judgmental about some of the choices my mother had made about her life,” she said. “But then she wrote all about herself in the book and I began to understand a lot more about her choices and opinions. That understanding made me drop my judgments and just accept her for who she was.

“What I learned is that we change when we talk to each other,” she added. “These books aren’t just about what is written in the book but about the way families begin talking to each other afterwards. I got so much more than the filled-in book back from my mother – I got a new connection with her. We talked on the sofa for the first time – sitting there for an hour, just chatting.”

Elma learned stories about her mother and about herself that she had never known before. “The most heartwarming story was how my mother had let me walk to school on my own when I was five years old,” she said. “At least, that’s what I thought she’d done. Apparently as a child, I’d begged and begged her to let me walk the short distance to my new school on my own. She finally agreed and I felt so excited and such a big girl! It was only when I read the book that I realised she’d secretly followed me, hiding behind trees whenever I looked around.”

Elma said that some mothers have taken seven years to fill the book out while others blitz it in a single weekend. Some find they have so much to say that they add extra sheets of paper, make drawings or tuck newspaper clippings and photos into its cover. Some mothers and daughters fill it out together – one daughter bought it to her mother’s bedside when she was seriously ill in hospital, successfully transporting them both away from a distressing present to a love-filled past. Five sisters on a weekend away together decided to fill out books for their own children – discovering as they sat and wrote them together that they had completely different memories of their childhoods.

One daughter discovered her dying mother had spent her last illness secretly filling out the book for her. She read from the book at her funeral. “She now says it’s the most valuable possession in her life,” said Elma. “And it is hers, too,” she added. “Once the wrapper is taken off the book, my name disappears from it. This book belongs to the person who fills it in.”

I judged her but then I began to understand her choices and views

Elma believes that modern life mitigates against the sharing of family histories and that subtly cuts us adrift, not just from our families but from ourselves. “I think times have changed so fast over the past few years,” she said. “It’s become so hectic and busy, and we are all looking for meaning and achieving goals and trying to keep a grip. But for me, it’s become really simple: what makes you happy is being connected with yourself – and being connected with yourself means being connected to your parents and grandparents.”

Relationships give life meaning – and stories are what those relationships are built on, Elma maintains. “Nowadays, a lot of stories get lost because they’re not shared. We have pictures, but not stories. Facts but not background. It’s important and wonderful to understand where we come from, but over time, we have forgotten how to connect and share those stories. We’re just too crazily busy to pass on the stories that matter.”

Social media, Elma believes, distracts us from telling each other these stories by offering an endless stream of superficial anecdotes. “The online world is where you communicate in fragments and where people show an image they want you to see,” she said. “But that communication lacks emotional depth.”

Since sales of Mum, Tell Me, soared, Elma quit her job as a telecoms executive to “follow my heart by making books that I hope will help others tell the stories that really matter”. She now does public speaking, kicking off by asking questions of her audience about their mothers. Most, she said, don’t know the answers. “Some people just never ask,” she said. “It doesn’t mean they don’t have a good relationship with their mother; just that they haven’t felt the need or had time to stand still for long enough to ask them.”

Margreeth ended up defying the doctors, dying last year, 14 years after her terminal diagnosis. “When the books were published in other countries, it was not only my adventure, but also hers. We thought it was so wonderful that we got to share our story, hoping to inspire others with it,” said Elma. “Although I am sad and I miss my mother every day, I am so much happier than when I was a successful businesswoman because I realise what success means to me. For me, it’s knowing what really matters in life and that is not the things you have but having a true connection with yourself and the people who are most important to you.”

Mum, Tell me by Elma van Vliet is published by Penguin at £12.99. Buy it for £11.43 at guardianbookshop.com