Nutshell by Ian McEwan
The Sunday Times Number One Bestseller Trudy has betrayed her husband, John. She's still in the marital home - a dilapidated, priceless London townhouse - but not with John. Instead, she's with his brother, the profoundly banal Claude, and the two of them have a plan. But there is a witness to their plot: the inquisitive, nine-month-old resident of Trudy's womb.