Truly, Madly Guilty by Liane Moriarity

Three couples gather at a barbeque and something happens that changes all their lives.  The story is slowly revealed through all six voices both around the event itself and in real time months afterward.  Six voices in two time periods actually wasn't as hard to keep track of as you may think but makes it difficult for start and stop reading.  Erika and Clementine have been friends since they were children but both have very different and mostly unshared views of that friendship.  In fact much of the real feeling among various members of this group is hidden - between husbands and wives as well as between friends.  About half way through the book, you learn what happened but it is this slow examination of the real inner lives that is the heart of the story.  Through in various forms of guilt about a neighbor's accidental death and you have an engaging easy read.