The book "Astrophysics of the Diffuse Universe" offers a comprehensive analysis of the physical principles that have contributed to the development of our understanding of the interstellar medium. The authors, Michael A. Dopita and Ralph S. Sutherland, present the complex phase structures, physical conditions, and chemical compositions of the diffuse universe in a clear and accessible manner. This work is aimed at postgraduates, advanced students, and professionals in astrophysics and serves as a valuable reference that provides a solid foundation for understanding the matter in interstellar space. The authors have adopted an educational approach based on their experiences from teaching courses, avoiding excessive mathematical complexity to convey fundamental physical insights to the readers. The topics covered are structured to progress from simpler to more complex systems, facilitating the understanding of matter in the universe.