The shack is an excellent introduction to the Holy Trinity.  While it is a piece of fiction, the content and main idea draws us in, with the main character, through the stages of grief and calls us to allow the trinity to move us toward transformation, begging us to love our enemy, to find some way to forgive, let go and allow our hardened hearts to open to the gifts that get locked out when grief and loss take over and are unprocessed.
I would recommend this book to someone who wants to see the way people and circumstances of our lives bring us closer to God.  Even when we are so angry and so riddled with grief that our faith is lost. This book illustrates how people in our lives can be sacremental to us, bringing us back to God, and to a place where our wounds can be healed and suffering can lead us to transformation.
My favorite quotes from this book are:
"He can speak intelligently about most anything, and even though you sense he has strong convictions, he has a gentle way about him that lets you keep yours."  pg 10
"I suppose that since most of our hurts come through relationships so will our healing, and I know that grace rarely makes sense for those looking in from the outside."  pg 11