The bible for raytracing techniques is PBRT, which, funny enough, is also now available online: http://www.pbr-book.org/
It is a pretty fantastic time for easily available, high quality resources on rendering in general and ray tracing in particular.
Its langauge neutral, which is interesting.
I had no idea he'd written more. Going by author alone, this book is a must-buy.
It really shows how simple the core concept is. All of these programs are well under 300 lines.
Faster ray tracers have better data structures and more bsdf functions. But regardless, the real benefit of raytracing is the simplicity of the concept.
The author has also written an impressive OpenGL-like renderer in just 500 lines: https://github.com/ssloy/tinyrenderer/wiki
It's a good thing I have coffee then. As someone who is not very familiar with computer graphics programming, I did enjoy reading through this.