What is the theme in A Certain Slant of Light by Laura Whitcomb?

The primary theme in this book is dealing with life after death. The main characters are ghosts and spirits. James is a ghost in a human's body. He is able to see other ghosts and recognizes that there are two different kinds of spirits. Some are the ghosts of people who realize that they are dead, and other ghosts wander around not realizing that they have died. He notes that Helen is unique. She sees herself as Light.