Yeah it was a great learning tool. pbrt is a good learning tool, but sadly it already implements most of the basics. Courses at Cornell and ETH Zürich used a small rendering framework called Nori [1], which is a pretty good learning tool. Shadertoy is nice, but perhaps a bit annoying to work in (as it involves a lot of tricks). I guess the best way to learn is to write your own ray tracer, but that takes a lot of time.

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