How to Use Blender's new ULTIMATE Shader: Principled — Blender Guru
Principled BSDF IOR Fix - BlenderNation
Basic Principled BSDF – Blender Knowledgebase
Principled Volume — Blender Manual
Anisotropic BSDF — Blender Manual
Principled BSDF is based on PxrDisney - PxrDisney is legacy - Blender and CG Discussions - Blender Artists Community
Here's a free Principled BSDF Shader guide to help Blender users, an extract from the free PDF guide and YouTube tutorial below, hope you find it useful :D : r/learnblender
Disney Ubershader now in master! ("Principled BSDF") : r/blender