Not to be pedantic but "ray tracer" evokes a concept that is a fair bit narrower than a "path tracer." The latter has concepts of sampling multiple directions and attempts to do things like conserve energy, respect the statistical properties of the surfaces it interacts with etc. A perfect ray tracer is easy to write but untenable for most real-world usage so PBRT is written more to the tune of all the various things you can do to improve performance without introducing statistical bias.