What is the theme in I'd Tell You I Love You, but Then I'd Have to Kill You by Ally Carter?

One of the most important themes in "I'd Tell You I Love You, But Then I'd Have to Kill You" is the concept of friendship and sisterhood as evidenced by Cammie's close relationships with her friends. At the beginning of the novel, Cammie is eager for her classmates to return to school and she does not feel like she is really home again until they join her at Gallagher Academy.