Lionel Shriver's gimmicky new novel, The Post-Birthday World, takes as its focal point the emotional life of Irina McGovern, an American ex-pat living in London with her longtime partner, Lawrence. The two have never married, but share a comfortable middle-class existence based on mutual affection and respect (though their sex life is perfunctory). One night, Irena goes out to dinner with an old friend—snooker player Ramsey Acton—to celebrate his birthday, and there's a moment after dinner in which she finds herself wanting to kiss him. At this point, the book splits off into two narratives: one in which she does kiss Ramsey, beginning an affair that culminates in her leaving Lawrence; and another in which she does not, and instead returns home to rededicate herself to her relationship with Lawrence.

The character of Irena is well drawn, and on some level it is interesting to consider how one woman can respond so differently to two different men: The Irena of the Ramsey narrative acts nothing like the Irena of the Lawrence narrative, yet she is still fundamentally the same character.

In general, though, the para-llel plot structure grates more than it illuminates. Rephrase the plot and you've got a pretty standard romance-novel formula (a still-sexy woman bursting with ripe unrealized sexuality leaves her stodgy husband for a dashing, devil-may-care snooker player...); maybe Shriver's structural hijinks are an attempt to distance herself from that tawdry genre. Instead, the gimmick only disrupts the continuity of the universe Shriver has created. As much as the reader longs to identify with the likeable Irena, it's hard to invest fully in the reality of the novel when that reality is abruptly bisected (and when too-precious reminders of the parallelism keep cropping up—Irena eats at the same restaurant with Ramsey, then with Lawrence. Vive la difference.). The best parts of the book stem from Shriver's very traditional strengths: She places interesting, multidimensional characters into a believable reality, and shapes their lives with insight and sensitivity. Unfortunately, instead of allowing these elements to sustain her novel—which they very well could—she's thrown in a predictably alienating po-mo gimmick that undermines the very talents that make her writing worth reading in the first place.