Never having heard of Lionel Shriver, it was the premise of THE POST-BIRTHDAY WORLD that intrigued me. The plot revolves around the consequences of a single choice, presented at the end of the first chapter. In this case, the choice is whether or not Irina McGovern, a 43-year-old children's book illustrator living in London, will succumb to temptation and kiss the dashing, somewhat louche professional snooker player, Ramsey Acton. The choice is momentous because she has lived amiably with Lawrence Trainer for the past 11 years, and she is not the sort of person to throw away a hard-won, cozy (if predictable) relationship at the whim of passion.

In alternating chapters, then, we follow two paths --- one in which Irina follows her adulterous instincts, and one in which she doesn't. Such is the author's astute knowledge of human nature that in each case Irina occasionally longs for what "might have been." Each alternating chapter covers the same period of time, and certain events have a pleasing way of echoing through. Some are large, like Irina's first children's book that she writes as well as illustrates being nominated for a prestigious prize. Some links are subtler, like the way the same girlfriend reacts when Irina confides in her. "Lawrence is a good man, Irina. They're thin on the ground. Think twice," she says, at news of Irina's dalliance. Yet in the next chapter when Irina confesses that she was tempted but resisted, the friend laughs. "Why didn't you do it? Might have been good for you!"

As Irina hangs out with Ramsey's snooker crowd, she misses the serious discussions of Lawrence's think-tank friends. But while socializing at a post-lecture party as Lawrence's partner, she wishes they could talk about something a little less weighty --- snooker, for instance.

The ramifications of each decision are fully explored as years pass, both choices bringing unique rewards and downsides. Shriver is not only witty but wise and at times profound, with all manner of metaphor to savor. "He [Lawrence] had a tendency to talk feverishly all around the main thing, as if bundling it with twine. Presumably if he talked in circles around the main thing for long enough it would lie there, vanquished, panting on its side, like a roped steer." That's only the first of more than 30 passages I marked, on my first time (there will be more) through the book. Here's another: "Lovers communicate not inside sentences, but between them. Passion lurks within interstice. It is grouting rather than bricks."

THE POST-BIRTHDAY WORLD is a long novel, with lengthy chapters, bursting with insight, humor and British slang. (After finishing the book I heard the author say in an interview that one gets two novels for the price of one.) Irina and Ramsey have great sex and spectacular "rows." During an escalating fight while preparing a rare home meal, she asks, "Why should I cook as if I'm at a Zen ashram just so you can touch base with a conversion on the road to Damascus with some ass-kiss vegetarian slag?" And Ramsey, who dropped out of school to perfect his snooker game, replies "Damascus?"

Certain tampon and sex-related details convinced me that Shriver employed an uninhibited female spy, or retained access to memories of a previous life as a woman. So I shouldn't have been surprised to turn the final page and see the author's photograph on the inside back cover. But I was already reeling from the deliciously subtle ending, and this pensive female face was a pleasant shock. On the Internet I discovered that Lionel changed her name from Margaret Ann when she was a teenager and that, like Irina, she's an American expatriate living in London. Her novel, WE HAVE TO TALK ABOUT KEVIN, won the Orange Prize for Fiction, and she has written six other books.

One of them is already on its way to my mailbox.