When the husband or wife of a bestselling writer publishes a novel about a troubled marriage, people are bound to ask if it is based on real life. Isabel Fonseca is married to Martin Amis, she is about the same age as her main character and they are both American. The parallels do not end there: Fonseca is a literary journalist and she has made her heroine, Jean, a successful columnist on a magazine.

Jean's subject is health, but the similarities are close enough to have sparked speculation about the novel's inspiration and whether it is a roman à clef. Outside gossip columns, the question does not matter much because it has to stand or fall as a piece of fiction; all novels are a blend of the author's imagination and observation of real events. Not every author who tackles an unhappy marriage has been in one, although he or she may have seen someone else's at close hand, and Fonseca has dedicated the book to Amis.

Jean's husband is physically unlike Amis and he is an advertising man. The couple have been together many years, have a grown-up daughter and are affluent enough to own a house in London and another on an island in the Indian Ocean. These are not the lives of ordinary people, but their escape to paradise puts Jean in the familiar position of a middle-aged woman who suddenly has to reassess her marriage.

Early in the novel, she opens a letter addressed to Mark and finds an invitation from someone called Giovana to embark on an erotic e-mail correspondence. Instead of confronting Mark with evidence of what appears to be an affair, Jean visits a cyber cafe and answers the letter herself, pretending to be her husband. Inevitably, this subterfuge accelerates their estrangement, providing Jean with copious material that she uses to create a lurid mental picture of her husband's infidelity.

But Fonseca also seems to be suggesting something else about the impersonation: that it allows Jean, whose prosaic name has clearly been chosen deliberately, to explore an undeveloped side of her own character. Careless until now of her appearance, and perhaps even a little self-hating, Jean is fascinated by the exuberant vulgarity of Giovana; she is far from being the first person to use the internet to play erotic games, even if the consequences on her return to London are comic and unexpected.

While all this is going on, Mark gets on with his chaotic life, rushing off to overnight business meetings and fuelling Jean's fantasies about the affair. Fonseca has previously written an acclaimed book about the Roma, but this is her first novel, and Mark's erratic presence prevents him from emerging as a fully drawn character. That is always a risk in a novel that relies heavily on the main character's habit of introspection, and the reader ends up knowing much more about Jean's internal world than the other characters' thoughts and feelings.

Outside events take on the hue of catalysts for Jean's emotional journey; in a novel stuffed with rites of passage, she has to contend with her daughter's first serious boyfriend, her father's life-threatening illness and a health scare of her own. Then a visit to New York to see her father in hospital puts temptation in her path in the form of a former boyfriend; Larry is a successful lawyer, a serious intellectual and about to get divorced - the perfect man, in other words. He is also undemanding, making Jean feel desirable and giving her a sense that she has choices.

There is every indication that this is a serious novel, and that Fonseca intends it to be more than a light read. She has an expressive turn of phrase and a gift for evoking a sense of place, but her final revelations (including the twist on which the plot depends) are unlikely to come as a surprise to the alert reader. Attachment exhibits most of the characteristics of popular women's fiction, from the heroine's self-obsession to its consoling reflections on middle age - Sex and the City for grown-ups, in other words.

Attachment by Isabel Fonseca
Chatto £15.99 pp320

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