Fiction by Jane Green.
(image from goodreads.com)
This was a good book, but I suffered from Oh-No-I-Can't-Look Syndrome through the middle of the story.
That's when you know a main character is making a major error in judgement that's going to have huge and terrible consequences.
In this case, she allows a person into her life that you just KNOW is going to wreck it. (I don't consider that information a spoiler when the front cover of the book says "Sometimes the perfect marriage can also be the perfect target.") Still, it comes out all right in the end. Mostly.