So we meet Cassandra Mortmain, frank, sharp and adorable narrator of I Capture The Castle, already engaged in two of her three main activities: diligently working her prose style while improvising around the cramped poverty of her situation.

I Capture The Castle is written in the form of a diary, and Dodie Smith adheres to this commitment with an integrity that becomes alchemy. Some writers may use a diary as a conceit while paying little attention to what it means, but Cassandra's diary is a physical thing. She has to find places to write it; it has finite pages and she can't afford more (and in fact she moves from book to book as the story progresses and people give her new diaries); life and its demands press on her even as she writes. The very act of addressing the reader captures Cassandra in motion: her writing isn't just a description of her life but an active part of it. To read the book is to meet the living girl.

What kind of girl is she? The first sentence tells us several things. 'I write this', she says, using the educated 'write' rather than the more colloquial 'I am writing': there's class here somewhere, a feel for the literary use of language. Class, but no luxury, or else she'd surely be writing somewhere more comfortable - and no pretensions either: she's not above sitting in the kitchen sink, and she's not above telling us that she's doing it either. Youth and physical health are implied too: it's a spry young thing who'll settle down into a sink for a long writing session. Cassandra is cultured, but she's utterly fresh; what in old-fashioned books might be called 'unspoiled'.

Her culture comes across all the stronger for this background: this is no hothouse flower whose education is merely imposed on her for the sake of convention. Cassandra and convention are clearly strangers. Instead, we are presented with a fine flower growing in rough soil, working hard on culturing herself. She may have to sit in the sink to get enough light to write by, but that doesn't hinder her writing style or her honesty. Cassandra's writing and intelligence are part of her, and difficulties will not undermine them. The image is comical, almost slapstick, but her direct, practical rendering of it makes us laugh with her, not at her. It's a rare girl who can retain her dignity sitting in a sink, but Cassandra's is actually enhanced by it.

Writing and growing in rough soil: these are two of Cassandra's talents. Her third talent, loving people, will begin to emerge fairly quickly as she favours us with her undeceived but kindly portraits of those around her, but we begin centred on Cassandra, on her relationship with herself - or rather, her relationship with her own ability to make something out of her circumstances. The book will have a rather melancholy ending, in fact, but we leave it warmed nonetheless, and it's Cassandra's energy, her vibrant sensitivity and survivor courage, that warms us. That begins here. Other people may let her down, but she will always have herself, and we know this from the first sentence.