This is pretty similar to Peter Shirley's "Ray Tracing in a Weekend" series, which gives some more detail and discusses the ideas more, and which recently was put online as "pay what you want": https://twitter.com/peter_shirley/status/984947257035243520?...
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.
This is by Jamis Buck! He wrote Mazes for Programmers, one of the best programming books I've ever read. It is clear and well-explained; you could write the code from the text description without seeing the code samples, which is incredibly rare.
I had no idea he'd written more. Going by author alone, this book is a must-buy.
Smallpt is another project which has been written and rewritten to demonstrate raytracing.
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 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.