I have always taken a pleasure in Carol Shields's novels that was slightly indistinct. Perhaps it was her composure - the membrane of absolute competence around her prose that seemed also to be a form of reticence - which rendered the soul, the motivation of her writing, opaque. That isn't a criticism: reading Shields is like talking to a good friend, someone reassuring and wise who, out of modesty or sympathy, keeps her own heart a secret. Like Jane Austen, Shields is a mysterious presence in her own fictions, a sort of shaded figure, and it seems to me that, with this remarkable novel, her narrative has finally turned to that figure and unveiled her.
Unless is a formidable meditation on reality: it takes the vessel of fiction in its hands and hurls it to the floor. Shields's unambiguous prose is here put to the service of her intellectual daring, and the result is a book that speaks without pretension about its strange and singular subject: the relationship between women and culture, the nature of artistic endeavour, and the hostility of female truth to representations of itself. I don't think I have previously read a work of literature that succeeds in bringing that last point to life. It might be said that a woman cedes cultural centrality the closer she comes to the fact of her sex, and likewise that a great woman is never more than an impostor.
Reta Winter is a writer. But she has other things on her mind. "It happens that I am going through a period of great unhappiness and loss just now," she states in her opening line. We are, then, already far from that other country, that cultural centre where women spend all their ingenuity in the hope of remaining. Reta's past literary achievements include, as well as "light" fiction, a career of service to Danielle Westerman, poet and feminist thinker, whose considerable works she has, over the years, translated from the French. Danielle Westerman is Simone de Beauvoir's spiritual daughter. She has lived the ascetic life of the female intellectual, free of fleshly association. She is hanging on by the skin of her teeth to her place among the male monuments of literary achievement.
Reta herself has not lived such a life: she has a husband and three children, and is, as she tells us, going through a period of great unhappiness and loss. Her eldest daughter, Norah, is what she has lost - not to death, but to a mute estrangement. Norah has dropped out of university, and sits begging on a street corner in Toronto. Around her neck hangs a sign on which is written the word "goodness". Danielle Westerman's theory is that Norah "has simply succumbed to the traditional refuge of women without power: she has accepted in its stead complete powerlessness, total passivity". Reta sees in Norah's protest her own despair at the futility of female endeavour. "My heart is broken," Reta writes on the wall of a toilet cubicle in a Toronto restaurant. In public, she cannot be so concise.
Though in pain, she continues to live, while on the street corner snow falls and the temperature drops far below zero. She meets her women friends weekly for coffee: this used to be a writers' group, but now it's more like a service station on the compassionless motorway of experience. In her spare time, she picks up her pen without relish, and tries to write the sequel to the novel she wrote before her daughter left her. She writes letters, too, the sort that don't get sent. They are addressed to people whose habit of conferring cultural invisibility on women has come to her attention. She submits her work-in-progress to her editor - who says that, if she consents to shading the heroine into the background of the novel while bringing the principal male character centre stage, they might well have a masterpiece on their hands.
For a moment, it seems that Reta will walk through the broken window of her sex, but in the end she does not. Her emotional life, after this long period in abeyance, is brought back into flow by a series of chances. Her daughter is retrieved; her novel remains "light": she is reincor-porated into that invisibility, ploughed back into that fertile, unexceptional loam of female being. And yet this is itself a novel. Are we witnessing an act of self-immolation, of martyrdom? I don't think so. Shields has produced a very, very clever book about motherhood, honour, art, language and love. It is a lament, a punch in the face, an embrace. It is, in a strange way, a feat of translation. I want to call it a masterpiece - but I think I'll leave that for a man to say.
Rachel Cusk's most recent book is A Life's Work (Fourth Estate, £12.99)