Love the One You're With by Emily Giffin (2008, Hardcover)

Ellen Graham is married to Andy, who is her best friend's brother. She feels she has the perfect relationship and will be happy forever after. But one unpredictable afternoon, she runs into her ex-boyfriend, Leo, who she hasn't laid eyes on in 8 years. Despite Ellen's love for her husband, she begins to wonder an age old question whether she's living the life she was meant to have.
I think the book points out that we all question at times whether we are with the right person, or have the life we really want....nothing is perfect, but we think there is a small chance it could have been better. The old adage of the grass being greener on the other side.....it never really is. Ellen and most of the other characters are stereotypical of what you'd expect in this story but it is still well written and Ms. Giffin's fans will not be disappointed.