The first, startling epigraph in Nicci Gerrard’s new book, “The Last Ocean,” comes from Emily Dickinson: “Abyss has no Biographer.” Gerrard sets out to tell the story of dementia, a disease that can appear to consume those it afflicts. After her father, John, died in 2014, the author — who writes best-selling thrillers with her husband under the name Nicci French — embarked on learning more about the disease as both a journalist and an activist. The result is a tender, inquisitive tour of a subject that can be raw and painful. Below, Gerrard talks about loss, art that punches you in the solar plexus and the experience of writing a book that doesn’t answer questions.

When did you first get the idea to write this book?

I first had the idea when my father, who’d been living with dementia for over 10 years, went into hospital in February 2014. After four weeks without anyone to see him — we were allowed in for very limited times and then not at all, because of a norovirus outbreak — I barely recognized him. I will, for the rest of my life, feel terribly that I didn’t get him out earlier. Then he lived at home for nine months. He had become skeletal, immobile, inarticulate, and in a way he felt utterly lost, like a ghost in our lives and in his own life. He would lay downstairs in a hospital bed, looking outside at the garden he used to love.

There was this clear sense that he’d already lost everything he had, everything he was, all his capacity, there was nothing left — and yet somehow that he didn’t lose himself. In the book I say that if I were religious, I would call that self he retained his soul. Something very indelible remained.

And witnessing that, I knew there was a book I could write about how mysterious it is to be human, really. But I absolutely didn’t think there was a book I would write. I didn’t want to write a memoir; my father was a very private man, and my family’s quite private, and there are lots of wonderful memoirs. So instead of writing what I’d been through, I launched this campaign for better care for those who go into hospital. For three or four years, I spent my working days talking to doctors, nurses, carers and, above all, people living with the illness. I knew I had to find a way of making that into a book full of lots of different voices and stories. I didn’t want to write a book that was certain and had answers. I wanted to write a book that was full of questions and feelings.