By Jennifer Reese
Updated March 16, 2007 at 04:00 AM EDT

Should you stick with your stable, unthrilling relationship, or succumb to a sexy new lover? Do you honor commitment, or a yearning heart? The central quandary of countless country songs, fatuous self-help books, Anna Karenina, French movies, and fretful late-night conversations may never have received a more thorough and nuanced treatment than it does in Lionel Shriver’s outstanding new novel, The Post-Birthday World.

Shriver’s shy, winsome protagonist, Irina McGovern, is a children’s book illustrator enjoying her ninth year of staid monogamy with Lawrence, an earnest, slightly prudish policy wonk at a London think tank. Whenever Irina has a cigarette or a second drink, he complains. But they have sweet nicknames for each other, a steady, if routinized, sex life, and cozy rituals, like a nightly bowl of popcorn, which, unlike their lovemaking, Irina keeps well spiced. They also hold an annual birthday dinner with their pal Ramsey Acton, a rakish professional snooker player who ”had a way of looking at Irina and only at Irina that no one had employed for a very long time.” Principled but maddeningly naive, Lawrence insists that Irina uphold the tradition when he’s away on business. Reluctantly, she does so, and after a languorous night lubricated with plenty of sake (no complaints from Ramsey), she impulsively decides to…?

At the end of the first chapter, the narrative branches. For the next 470 pages, in alternating chapters, Shriver meticulously plays out two scenarios: In one, Irina embraces temptation, and a chaotic, erotically charged life with impecunious Ramsey; in the other, she resists, congratulating herself on her virtue and settling back into her easygoing rapport with Lawrence. The well-worn fork-in-the-road concept could easily have yielded a sterile exercise, but Shriver, a brilliant and versatile writer, allows these competing stories to unfold organically, each a fully rounded drama, rich with irony, ambiguity, and unforeseeable human complications.

Shriver resists stacking the deck. Each of Irina’s men, and each of her possible lives, has appeal; each has profound limitations, and writing about them, Shriver teases out a knotty set of questions: Does happiness reside in everyday contentment or passionate connection? How important is sex? By the same token, how important are shared values? The novel will provide juicy fodder for animated book-club conversations: The answers fluctuate page by page, and, like a Rorschach test, the clues will be interpreted differently. Until, that is, the very end, when Shriver comes down firmly on one side.

Or at least that’s how I read it. A