What is the theme of book p.s. Longer letter later?

P.S. Longer Letter Later is a collection of letters written by a young woman named Lily to her friend Emily, who has moved away. The letters cover a wide range of topics, including Lily's day-to-day life, her thoughts and feelings about her friends and family, and her struggles with mental illness. The book's themes include friendship, mental health, and the importance of communication.