Pixar has ditched RSL in favor of BSDFs written in C++, so there is no real reason for learning RSL anymore. Open Shading Language (OSL) is a new shading language which is getting more and more use in the industry, but it's completely different by design.
PBRT is a great resource too. I don't know if they opted out of literate programming? I can't stand that for some reason. I've leafed through PBRT though. I've learned (back in the day, hah) from Shirley's Ray Tracing book and bibles that Andrew Glassner wrote (Principles of Digital Image Synthesis) which I still consider awesome resource.
The current edition of Physically Based Rendering still uses literate programming, but most of the maths are separated from the code, so you can skip over most of the code. Even if you can't stand I can really recommend reading it, as it's a great book. Shirley's Realistic Ray Tracing is still very useful these days, but obviously it's lacking the state of the art. If you really want to avoid any code you should read Veach's PhD thesis [2] and then some papers on BSDFs [3], shapes [4] and ray tracing acceleration [5].
[1] http://wjakob.github.io/nori/
[2] http://graphics.stanford.edu/papers/veach_thesis/
[3] http://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.html
[4] http://www.graphics.cornell.edu/pubs/1997/MT97.html
[5] http://www.nvidia.com/object/nvidia_research_pub_012.html
I've written an RSL tutorial that has been online for past 15 years (it's currently offline, I'll put it up again) and I've had people from all over the world from different studios (big and small) thank me.. it seems RSL doesn't have much resources online and considering this is tied to Pixar - do you have plans for RSL?