John Carmack's 2013 QuakeCon talk, Physics of Light and Rendering. This is the single best introduction to the history and current practices of shading I've ever seen... probably because it's being delivered by the guy who invented half of it.
Siggraph 2014, Physics and Math of Shading. Good intermediate level explanation of PBR and shading theory in general.
Physically Based Rendering Encyclopedia. Compiled by 3py0n over on Polycount, this is a collection of a whole bunch of introductory documents related to PBR.
Allegorithmic's Theory of BPR Document. A lot of this data is in the PBR Encyclopedia, but this is well laid out and looks nice, and so might be a better reading experience (or just a good second opinion).
Disney BRDF Explorer. This is a great tool for comparing various shading models to ground truth scans of materials (though reading the output can take a bit of getting used to...
BRDF Lab serves a similar function, but I haven't used it yet. It's open source, though, so that's cool.