I just finished a great book called "The Post Birthday World." It's about an American woman living with her partner in London, and the first chapter ends with her being tempted by another. From there, the chapters alternate between her life proceeding after she gives into temptation and her life proceeding after choosing to be faithful. It's sort of a parallel universe thing, and it is very interesting to see the same situations played out differently based upon her choices. It was difficult to put down, because I wanted to get to the end to see which ended up being the better choice for her.
I can't remember the author's name, but she is the author of "We Need to Talk About Kevin."