General computer graphic courses likely applicable to Houdini. Title image by Keenan Crane / Carnegie Mellon University.
Three consecutive books on ray-tracing (Code -> Download ZIP):https://github.com/RayTracing/raytracing.github.io
A technical video series on ray tracing: TU Wien Rendering / Ray Tracing Course
Comes with an examplary ray tracer, as well: https://github.com/8BitRick/smallpaint