Comprehensive PBR guide by Allegorithmic

Allegorithmic, creators of Substance Designer (texture creation tool) have declared 2015 “The year of physically-based rendering” and they’ve published this free book about how it:

If you have seen our PBR demo, then you’ll see that we agree. Bring on 2015!

3D Coat teases PBR as well

As does Sketchfab