Book of a Lifetime: I Capture The Castle, by Dodie Smith

I'd seen it on the shelf for years before I opened it: a well-thumbed, yellowing paperback that belonged to my sister, a Peacock logo indicating its suitability for so-called "young adults". The author's name was familiar from, of all things, 101 Dalmatians. The title, I thought, conjured up an adventure story about small boys playing war games.

Wrong. Within a paragraph of the famed first line, "I write this sitting in the kitchen sink," I was hooked. I was 14. Years later, after I've reread it time and again, I'm as addicted as ever to its delicate layering of naivety and profoundly sophisticated observation. If it seems slightly dated, perhaps that's because its style serves its substance – poetic, musical and never pretentious.

In the mid-1930s Cassandra Mortmain, aged 17, keeps a diary of her family's impoverished life in a crumbling Suffolk castle. The daughter of a failing author, with an elder sister determined to marry money to rescue the family from its penniless rut, she captures the castle and its eccentric inhabitants with phrases that tie an instant reef-knot round the reader's attention. This isn't just a book; it's a world, and to read it is to live in it. I don't know whether it's left an impact on my own writing; I should be so lucky.

Yet the author of this remarkable work wasn't a young girl experiencing initiation into love and lust, pain and champagne ("like very good ginger ale without the ginger"), but a woman in her forties, living in Hollywood and feeling desperately homesick for England. You can see why Hollywood wanted her: she has delivered perfect structure, with settings and characters so thoroughly created that you seem to breathe their air. And the book isn't really for "young adults" at all. It's brimful with emotional and erotic subtlety, which Smith allows to shine through without hammering it home. The story is about unrequited passion – but even when Cassandra notices that she is part of "a game of second-best" in which everyone is seeking consolation after failing to win true love, she never quite lets her feelings interfere with her sense of irony.

Amid all the lyricism and liveliness, you notice that though this all goes on just before the Second World War, the characters live in blissful unawareness of the future. Smith, writing in the late 1940s, knew what lay in store; we share only her unstated hindsight. Was she deliberately preserving images of a pre-war world that she knew was gone for good?

Whatever the truth, I Capture the Castle has that elusive quality: a heartbeat of its own. The open ending and its last haunting words keep on pulsing when you close the cover – and when you turn back to the beginning to start again.

Jessica Duchen's novel 'Hungarian Dances' is published by Hodder & Stoughton