[DevOfNull]

If anyone wants to make their own: The free e-book Ray Tracing Gems II [1] covers realtime GPU ray tracing with modern APIs and hardware acceleration, and has a chapter about spectral rendering (Chapter 42: Efficient spectral rendering on the GPU for predictive rendering).

[1] https://www.realtimerendering.com/raytracinggems/rtg2/

[superb_dev]

I would also recommend Ray Tracing in One Weekend [1] if you want a very quick introduction, and Physically Based Rendering [2] if you want to then go really in depth.

[1] https://raytracing.github.io/

[2] https://pbrt.org/

[jplusequalt]

Highly recommend Physically Based Rendering. As a book, Pbrt is to the study of path tracers, as Gravity is to the field of general relativity. Wholly encompassing and rigorous.

[kqr]

I can't take time to fiddle with raytracing (however much I'd want to!) but I skimmed the first half of that book and alias sampling is a very elegant and nice technique. Wish I had known about it earlier! It is useful in a far broader context than graphics.