For me, one of the most powerful fiction books ever has been The Horse and His Boy by C. S. Lewis. The reason? At a time when I was angry and wasn't using the Bible, God met me in the book I was reading; The Horse and His Boy. My favorite part is the point in the book when Shasta tells Aslan all that has gone wrong in his life. Aslan turns around and tells him why each of those things were blessings in disguise.
Someday, I want to write a scene like this in my book.