The specialized book "Teaching Mathematics Outside of One's Expertise" by Marc Bosse offers a comprehensive analysis of the experiences of teachers who instruct mathematics without having formal qualifications in the subject. Through a neutral-phenomenological research approach, it examines how these educators perceive their role as mathematics teachers and what types of identities can be derived from this. The study is based on empirical data collected through surveys and observations in mathematics classes. A central finding is that the lack of formal qualification does not necessarily determine the self-perception and professionalism of the teachers. The book provides a model of six identity types that describes the diversity of experiences and attitudes of the educators, thus offering valuable insights for practice and teacher training.