Background: Physics and Math of Shading
Introduction to Physically Based Shading in Theory and Practice (Video)
Microfacet Models for Refraction through Rough Surfaces
Understanding the Masking-Shadowing Function
in Microfacet-Based BRDFs
Real Shading in Unreal Engine 4
Moving Frostbite to Physically Based Rendering 2.0
Photorealism Through the Eyes of a FOX: The Core of Metal Gear Solid Ground Zeroes
Physically Based Shading at Disney
https://interplayoflight.wordpress.com/2013/12/30/readings-on-physically-based-rendering/
https://gist.github.com/AbstractAlgorithm/d0194f477adf829822ff
https://eheitzresearch.wordpress.com/415-2/
http://maverick.inria.fr/Publications/2009/DN09/VolumetricBillboards.pdf