Book excerpt: This book provides course material in theoretical physics intended for undergraduate and graduate students specializing in condensed matter. The book derives from teaching activity, offering readable and mathematical treatments explained in sufficient detail to be followed easily. The main emphasis is always on the physical meaning and applicability of the results. Many examples are provided for illustration; these also serve as worked problems. Discussion extends to atomic physics, relativistic quantum mechanics, elementary QED, electron spectroscopy, nonlinear optics, and various aspects of the many-body problem. Methods such as group representation theory, Green’s functions, the Keldysh formalism and recursion techniques were also imparted.