It took me longer than anticipated to read this, and at times it seemed longer than it needed to be, but it was well worth the time; The Post-Birthday World was one of the best books I read in 2008.
The opening chapter presents the premise, extended from the first sentence quoted above: Over a period of several years, it has become customary for long-term (unmarried) couple Irina McGovern and Lawrence Trainer to meet Ramsey Acton for dinner on his birthday, the sixth of July, even though what was originally a couples' outing no longer is, since Irina's friend and collaborator Jude Hartford, Ramsey's wife and the original arranger of their get-togethers, is neither Ramsey's wife nor Irina's friend and collaborator any more. But one year, Lawrence is away at a work-related conference, and urges Irina to meet Ramsey for the birthday dinner without him; after dinner, drinks, and a trip back to Ramsey's home to smoke a joint, Irina is tempted at his snooker table.
From that point on, each chapter except the last one is told twice - once as if Irina gives in to the impulse, and the kiss with Ramsey leads to an affair that ends her relationship with Lawrence, and then to Irina and Ramsey's marriage; and once as if she doesn't, and returns to her life with her partner Lawrence. In both versions, Irina frequently reflects on what she might be giving up with one man as her life moves forward with the other. It's an excellent framing device, and Lionel Shriver employs it well. Exploring the characters through their actions and feelings in both scenarios, over a period of several years, develops different dimensions, and helped me feel more more connection to and sympathy for them.
I liked the way that each chapter essentially related a similar plot scenario, but with differing details and twists depending on which future it was talking about. For example, Irina writes and illustrates a children's book. In one version, it's a creatively assembled two stories in one that doesn't make a lot of money, but is nominated for a major award. In the other, it's a different story in a different style, more commercially successful, and it's nominated for the same award. I also liked that I really had no idea which of the two versions of Irina's future might be the "real" one; both have their positives and negatives, which makes either direction plausible, and I found it difficult to favor one over the other. The final chapter - which, like the first, is only told once - wraps things up while maintaining that ambiguity. I realize that this very attribute might annoy some readers, but for me, it's what made The Post-Birthday World an involving, original, and memorable reading experience.