Laura Cumming, the art critic for The Observer newspaper in London, published “The Vanishing Velázquez,” about a lost painting by the Spanish master, in 2016. Her new book, “Five Days Gone,” is about a disappearance of a much different — and much more personal — sort. In 1929, when Cumming’s mother was 3 years old, she was kidnapped from a quiet beach on the coast of England. A few days later, she turned up unharmed. She grew up with no memory of the event, and wasn’t even told it had happened until more than 50 years later. In the book, Cumming attempts to figure out what occurred, and in the process unlocks a great deal of convoluted family history. Below, she discusses discovering a bag of photos that changed the course of the book, looking at her subjects with an open mind and more.

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

I was always fascinated by my mother’s disappearance, and the village involved and all the villages on this part of the coastline would tell us nothing. My mother ceased to be interested in it as her life went on, because it took her back to a lot of anguish about her childhood.

I had been thinking of it entirely in terms of evidence — letters, documents, certificates. But one day I was looking through the family photograph album, a modest little book with tiny black-and-white photographs from the 1920s and ’30s. I began to notice things I probably should have seen years before. There were no pictures of my mother before the age of 3, or after the age of 13. I began to think about who had taken the photographs, and why she was always alone, never with her father or her mother. I began to re-examine what I knew about my mother’s life in light of these images.

I began to do so when my mother turned 90, three years ago. I realized around this time that the story had exhausted itself for her. She had moved into another phase of her life. I had hoped that she would write this book; it contains many extracts of her own writing.