Chang Shau-jin Chang 
Introduction To Quantum Field Theory [PDF ebook] 

This book presents in a short volume the basics of quantum field theory and many body physics. The first part introduces the perturbative techniques without sophisticated apparatus and applies them to numerous problems including quantum electrodynamics (renormalization), Fermi and Bose gases, the Brueckner theory of nuclear system, liquid Helium and classical systems with noise. The material is clear, illustrative and the important points are stressed to help the reader get the understanding of what is crucial without overwhelming him with unnecessary detours or comments. The material in the second part ranges from variational principles to path integrals, discusses gauge theory, the renormalization group and classical solutions together with their applications.