Non-Abelian Hodge Theory and Moduli Spaces of Higgs Bundles Notes by Guillermo Gallego.

Lecture notes on moduli of Higgs bundles and the P=W conjecture Notes by Jochen Heinloth. 

AGQ Group Project Non-abelian Hodge Theory by Shing Tak Lam and Emanuel Roth.

Appendix by Oscar Garcia-Prada to Differential Analysis on Complex Manifolds 3rd ed by Raymond O. Wells.

Master thesis and notes by John Benjamin McCarthy.

A Gentle Introduction to the Non-Abelian Hodge Correspondence Notes by Alexander Thomas.

Abelian and non-abelian Hodge theory Expository article by Lukas Brantner.

The self-duality equations on a Riemann surface Seminal paper by Nigel Hitchin.

Higgs bundles and local systems Seminal paper by Carlos T. Simpson.