"Odd Girl Out" by Rachel Simmons is a book that explores the hidden culture of female aggression and bullying among girls, both in person and online. It examines the dynamics of female friendships and how girls navigate social exclusion, taunting, and relational aggression, ultimately offering strategies for girls, parents, and educators to address these issues. The book also delves into the impact of bullying on girls' self-esteem and mental health, while providing resources for intervention and support.