'The Post-Birthday World' is the latest novel from Lionel Shriver, author of the Orange Prize-winning 'We Need to Talk about Kevin'. This book is told from the perspective of children's book illustrator Irina, an American expatriate now residing in London with her long-term partner Lawrence.

They live a contented and unexciting existence, a partnership that involves comforting domesticity and not much in the way of romance. Lawrence is a terrorism expert, and works in a prestigious think tank. He is steadfast and intelligent, with the drawbacks of being intellectually condescending and seemingly devoid of fun.

They have been plodding along for the past nine years, with nothing rocking their relative contentment. But when Irina meets their mutual friend Ramsey for a birthday dinner, while Lawrence is out of the country, she is suddenly dangerously attracted to him. He is a world famous snooker player, impulsive, daring and passionate - the antithesis to Lawrence.

Irina is struck with the feeling that her life hangs in the balance, all resting on her decision to kiss him. The book splits into two narratives from this point, with alternating chapters detailing her life if she takes the plunge with Ramsey or stays faithful to old dependable Lawrence. Her two contrasting destinies are explored in great, almost obsessive, detail.

The parallel universe structure can become quite tedious. Shriver slams home the point of how different Irina's life is in each scenario. Events are mirrored in each world, and the outcome is opposite. This ranges from minor things, like what food she is eating, to how her partners interact with her difficult mother.

The reader only has to think what has happened, flip the events and then they know basically how things will pan out in the next chapter. This causes an overwhelming sense of repetition and coupled with the sheer size of the 600-page tome, makes it seem unnecessarily long.

Luckily her writing is well paced and engaging, and draws you into each story effortlessly. If only Shriver could have resisted the urge to include the strained snooker metaphors that litter the text.

Shriver does abandon metaphor at one point, to explain exactly the moral of the book. Irina has written a children's novel and is telling Ramsey what it means. "The idea is that you don't have only one destiny. Younger and younger, kids are pressed to decide what they want to do with their lives, as if everything hinges on one decision. But whichever direction you go, there are going to be upsides and downsides."

Despite the structural drawbacks, 'The Post-Birthday World' is still a thoroughly enjoyable read, exploring relationships in an interesting way. The 'what-if' in all of us will be satisfied.

Sarah McIntyre