The book "Quantum Theory of Magnetism" offers a comprehensive analysis of the phenomenon of magnetism from the perspective of linear response. It examines how magnetic materials respond to various types of magnetic fields, whether uniform, spatially varying, static, or time-dependent. This updated edition goes beyond previous versions by addressing not only the magnetic response but also the resistive response of magnetic materials. The book also includes numerous problems that enhance readers' understanding. The relevance of the topics covered in this book remains strong, as new magnetic phenomena with significant scientific implications and innovative applications continue to be discovered. This edition also takes into account the latest developments in research, including a new chapter on magnetic multilayer films, thus providing valuable insights for students and researchers in the field of magnetism research.