Who writes the introduction in the book, The Habit of Being: Letters, and what does she find so intriguing about the letters?

Sally Fitzgerald provides an introduction and states that she knows the author personally. Fitzgerald also provides a brief summary of Flannery's life, and also describes the delight and mystery that some of the letters contained in the book were written when Flannery was a young girl and had written them within the secrecy of her diaries.