Physics & Math in General

A. M. Polyakov, Gauge Fields and Strings

Bryce DeWitt, The Global Approach to Quantum Field Theory

John Collins, Renormalization

Carlo Rovelli & Francesca Vidotto, Covariant Loop Quantum Gravity

Thomas Thiemann, Modern Canonical Quantum General Relativity

Thomás Ortín, Gravity and Strings

Papers

Witten, The Feynman iε in String Theory

https://arxiv.org/pdf/1307.5124.pdf

Feynman Propagator of a Scalar Field

https://web2.ph.utexas.edu/~vadim/Classes/2016f/propagator.pdf