Book of a lifetime: I Capture the Castle, By Dodie Smith

When people ask if I've "always enjoyed writing", I tend to regard them with horror. Of course I haven't! Writing fiction is incredibly hard work. My ideal is to be paid, per word, to read. This preference dates back to my "early years" which were divided, as authors say on book jackets, between an off-grid sheep farm on a rain-sodden Exmoor river valley, and a gloomy villa in shrubby suburban Brussels.

This was the first half of the 1970s, a time when grown-ups were busy chain-smoking, consciousness-raising, in flapping flares and high-heeled sneakers and God knows what else.

The only way to get through it was to read. I can even remember my mother shouting up the stairs to our rooms when she hadn't seen any of us for hours, "Stop reading!"

My book of a lifetime, which I am going to have placed ceremoniously in my coffin, is one I first read in 1974, when I was nine. I have inscribed my name carefully in the fly of the Penguin paperback in a round hand my mother calls "piggy knitting." It is I Capture the Castle by Dodie Smith. I can't think why I picked this out in the English bookshop in Brussels: these days, the cover, showing a dank turreted castle sinking into a moat, looks forbiddingly bleak. This is a book I read almost every year (I discard those who claim to "re-read Proust in French" on their summer holidays, the liars).

But familiarity is not the reason why this sparkling tale of two sisters, Rose and the narrator Cassandra, and two handsome American brothers, a drippy swain called Stephen, a warm pink and gold stepmother called Topaz, and a dark tormented hermit of a father with writer's block (who once attacked the girls' mother with a cake knife) will stay with me always.

I think I became obsessed because the family is so brilliantly realised: eccentric and wretched but attractive, and most importantly, poor, an essential ingredient for romance. Clever Cassandra feels like the sister I never had until life delivered up a beautiful half-sister when I was 16. The sense of place is so strong – the kitchen, the tower, the mound – that I feel I spent just as much of my childhood in the castle as in Brussels or Exmoor.

I think the only reason I write at all is because I want to write a book that someday, someone will love as much as I love I Capture the Castle, which has seeped into my veins like damp into the ancient, chill stones of the castle. It is my forever book.

Rachel Johnson's new novel, 'Winter Games', is published by Fig Tree