PBR
Comprehensive reference on physically based rendering techniques
Learn OpenGL's tutorial on physically based rendering
Google's documentation on the PBR system on their Filament renderer on Android. Best in class documentation about high performance PBR lighting shaders
The classic Kajiya rendering equation paper introduced the "rendering equation" and the path tracing algorithm
Blog post series describes how spectral rendering can be implemented at a relatively low overhead (as compared to RGB rendering) and why that is beneficial
the Academy Award-winning Ph.D. thesis by Eric Veach. It starts from bidirectional light transport algorithms and introduces multiple importance sampling and Metropolis light transport
An advanced book focused on light transport theory for global illumination.
This tutorial will cover the basics of physically-based rendering such as reflection models (BRDF), volume scattering (phase functions), optical phenomena (dispersion and polarization)