What is the moral of the book Wintergirls?

The moral of Wintergirls is that healing is difficult but possible. Lia and Carrie both struggle with body image issues and their friendship turns toxic as a result. Carrie breaks off the friendship and eventually dies from bulimia, further adding to Lia's inner turmoil. Due to the loneliness and guilt, Lia succumbs to self-harm and depression.
As a "wintergirl," Lia struggles to appear normal as her mental health continues to decline. Her distorted perception of beauty represents the high expectations that many young girls strive to achieve. It fuels her self-abuse and starvation, which puts her life in mortal danger. If not for her supportive step-sister Emma and the self-realization that her weight loss will not ultimately satisfy her, then she would have ended up like Carrie. Lia takes the first step towards healing by seeking the help she needs. Her recovery is uncertain, but it's not impossible either.