Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Ring the bells that still can ring
Forget your perfect offering
There is a crack, a crack in everything
That's how the light gets in.
– From "Anthem" by Leonard Cohen
Leonard Cohen is the "Hallelujah" guy. This song is similar: inspirational, popular, basically everything Piper Kerman wants her book to be.
This is a song to inspire people who are going through moments of darkness. And spending a year in prison, well, that's pretty darn dark. Kerman has to do what she can to find those cracks in prison and let the light in wherever she can find it.