Twin sisters Cassandra and Judith, 24 years old, outwardly identical, are entirely dissimilar within, each the opposite half of one whole personality. At least, that's Cassandra's perspective: where Cassandra is self-destructive and chaotic, Judith has managed to become a fully functioning adult, stable and practical; while Cassandra cannot allow herself to be loved, Judith has become engaged to a dependable doctor. And so it is that after the worst year of Cassandra's life, since her sister deserted her in Berkeley to start afresh in New York, she drives across California to the wedding at the family ranch in the unrelentingly hot foothills of the Sierra Nevada.

With her travels a sense of doom about the forthcoming Happy Event. The novel's title is slightly misleading, as the focus is upon Cassandra before the wedding, intent upon derailing it. Each sister is anxious to protect the other from herself, which to Cassandra in her self-absorption means preventing Judith from seeking something so unremarkable as uxorious contentment  a betrayal of the expectations they were raised with, and of their twin relationship that "we have to protect at every turn from the menacing mass of cliches thrust towards us from the outside". Judith, typically, is rather more pragmatic about her counterpart: "All she wants to do with her life is lose it somewhere."

Dorothy Baker masterfully shows Cassandra trying to do just that, somehow remaining vile and lovable at once, her noxious despair laced with bleak humour and a lot of brandy. In very few strokes each character emerges: the reclusive philosopher father, the brightly and blindly cheerful grandmother, even the twins' turbulent late mother. Only Judith eludes the author slightly, but in reality the more private and practical do tend to recede when surrounded by dramatic, demanding family members. The familiar tensions that are apt to erupt whenever weddings are being planned are timeless; the only reminder that this novel is 50 years old is the shock caused by bikini-wearing.

Thanks to the tireless raising of the point by Howard Jacobson and others, it seems just possible that the notion is sinking in that not to like a novel because one does not find the central character likeable is not, actually, a sophisticated way of reading. I need hardly add that I wholly endorse this view  but this doesn't mean that I think there's something immature or unworthy in taking pleasure in the characters a writer creates, whether you're the reader or the writer.

I thought of this when I came across the father-figure in Cassandra at the Wedding: soaked in Hennessy, but entirely charming, he gave up his professorship in philosophy because "it irked him to have to meet appointments". His great achievement, it would seem, is the talent and devotion of his twin daughters, who, in different degrees, have taken his example and look at the world carefully, and measure their words. It didn't take me long to work out who he reminded me of: Mr Bennet  you know, Elizabeth's dad.

This could, in fact, be a skewed, modern take on Pride and Prejudice. It certainly is a dark comedy about marriage. Cassandra  so named because when she was born, she wailed like her namesake at the gates of Troy  has found out that her sister is getting married, and we join her as she drives from Berkeley to her father's "ranch" to arrive in time for the nuptials. It is quite clear that she is going to be causing, intentionally or not, some kind of mischief.

She and her twin, Judith, used to be inseparable, considering themselves, with some justification, to be above and beyond the lot of common humanity ("We can start living where other imaginations fail," says Cassandra at one point, and the words both impress and chill). But Judith has been in New York for some months, with, as we learn, unhealthy consequences for Cassandra. And it is the actions of supremely intelligent neurotics we should be most alarmed of, or braced for. (Her bottle of sleeping pills says "as needed for sleep": as for her uppers, she says she understands why pharmacists don't write "as needed to encourage the minimum of tolerance for the brute stupidities of this world".)

So, Cassandra certainly is a character, and while she is monstrously selfish, and self-absorbed, and something of a menace to herself and those around her  she sure can tell a story. And you get her. You can tell from the very opening words that this is a voice not like others' voices: "I told them I could be free by the 21st, and that I'd come home the 22nd (June)." A sentence, or two sentences, sort of, almost daring in their banality at first sight: but we are caught by that parenthesised month. It shows a mind pedantically quick to make imprecise things quite clear; a tone that already suggests she is, though, weary of having to state the obvious, and happy to skip on the conventionalities of formal style. And all so lightly done you hardly notice it. And it also shows that, however dark the material of the book gets at times, it is never too far away from comedy.

This perfect nugget of a novel is also never too far from depth either, and as Deborah Eisenberg says in her useful afterword, we are also  what with the drunken philosopher  nudged to think of Plato's boozy Symposium, in which the theory is aired that love is about trying to reunite ourselves with our lost halves. So: twins.

First published in 1962 (her first novel was Young Man With a Horn, about Bix Beiderbecke, but I never got on with that because of my deaf ear for jazz), modern readers will also relish the breezily accepted materialism, the pin-sharp portrait of a tiny part of society, as if picked out in Californian sunlight. It is also knowing and wise: a lesson in how wisdom is worth more than intelligence  not that intelligence is to be denigrated. Eisenberg says this book "should never be out of print", and I couldn't agree more. This novel, despite its specific setting, hasn't aged in the slightest: really good writing like this doesn't age. It's always up to date.