Novels about an individual falling in love with a family have a distinctive pedigree: Evelyn Waugh's Brideshead Revisited springs to mind, as does Alan Hollinghurst's The Line of Beauty. At their best, such novels provide bracing sketches of a family's myths, legends and brands of madness, while revealing the narrator's own flaws. They become more dubious when satire, as it often does, shades into something close to social climbing.

Rachel Cusk has played with this sub-genre before. In The Country Life, an odd young woman abandons her marriage to move in with the mad Maddens, eventually discovering a measure of humanity. In her new novel, the odd young narrator is Michael, a university friend of Adam Hanbury, whose family lives in a beautiful West Country house called Egypt. Invited to a birthday party by Adam's sister Caris, he is introduced to a madly bohemian set-up in which "it's the place that matters, not the people". Vegetarian sausages are eaten, fireworks are let off and Michael kisses Caris. The Hanburys are the sort of people who pronounce Mass as "marse", who don't go to the dentist, and who remain tempting subjects for a young writer to lampoon or adore.

Six years later, Michael is embroiled in an unhappy marriage with a woman whose open-handed, libidinous parents tangentially remind him of those of his old friend. Now a bearded lawyer working for a charity, he plays the violin in pubs on Friday nights to remind him he still has a soul. When the balcony of his Georgian house in Bath falls off and nearly crushes him, he decides to revive a friendship that has fallen into desuetude.

In the Fold follows the Brideshead pattern. Michael may be more sceptical than Waugh's Ryder, but he still gets swept up into lambing, Adam's boring marriage and the Hanburys' land-obsessed quarrels. The family's "magical quality" inevitably turns out to be largely illusory: Adam's father divorced his attractive first wife in order to marry rich Vivian, who has been paying for the upkeep of the Hanbury country house and farm and is now the owner. As a den of vipers, the family is no more than averagely poisonous, even when one child shoots his sibling with a crossbow. In the end, Michael's best hope of maturity comes from loving his small son Hamish, whom his departing wife calls his "mascot".

One does not read Cusk for her plots, or for attractive and engaging characters. What is both splendid and appalling about her is her prose. Metaphor piles on metaphor. Even when they are beautiful and apposite (she contrives a miraculous comparison of undergraduates to "a bank of flowers in season . . . a waving mass of stalks and roots were for a time obscured": airy, populous and right), her similes are mown down by her machine-gun approach. It is scarcely credible that In the Fold could have been longlisted for the Booker when it contains passages such as:

Hamish was a big, peculiar baby with flowing blond hair and the prominent features of a general or politician. He seemed to relish pointing out the obvious, and treating everything as a joke: in this way he was identifiably male, though in spite of his size and virile countenance there was something effeminate about him. He was like a big, exuberant, bad-mannered amphibian, or a laughing, androgynous cleric.

The first two sentences are charming; the last should get Cusk's editor sacked. Her characters speak in riffs of deliciously absurd affectation and unhappiness that approach, waveringly, the Chekhovian ideal. Cusk has written a genuinely funny comedy of manners, a small, spiky novel about the loss of autonomy. It is, as she might put it, "not unimpressive", but it could have been so much better had it been composed in a style that displayed, rather than betrayed, her intelligence and wit.

Amanda Craig's latest novel, Love In Idleness, is published in paperback by Abacus