The 3rd book [0] is primarily about implementing an MC ray tracer and then adding the techniques in the second paper you linked. That's what I've used in my code as well. I just skimmed through the paper and I think the book covers most of the material from the paper with the exception of spatial subdivision. I will have a more careful read later to see exactly what my implementation is missing from this.

[0] https://raytracing.github.io/books/RayTracingTheRestOfYourLi...