 Quantum Field Theory by David Tong

Authored Cambridge DAMTP lecture notes: www.damtp.cam.ac.uk/user/tong/qft.htm

Chapter 1, Classical Field Theory: www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S1.html

Chapter 6, Quantum Electrodynamics: www.damtp.cam.ac.uk/user/tong/qft/qfthtml/S6.html

Cited sections are 1.1, 1.3.4, 6.1.1 and 6.3.2. The current signs in any application must follow its stated phase and covariant-derivative convention.