The book covers it in Appendix A.6 (p 424) in the v2023.06.11a PDF file.
> A.6 What is the Difference Between “Concurrent” and “Parallel”?
> From a classic computing perspective, “concurrent” and
“parallel” are clearly synonyms. However, this has not
stopped many people from drawing distinctions between
the two, and it turns out that these distinctions can be
understood from a couple of different perspectives.
> The first perspective treats “parallel” as an abbreviation
for “data parallel”, and treats “concurrent” as pretty much
everything else. From this perspective, in parallel computing, each partition of the overall problem can proceed
completely independently, with no communication with
other partitions. In this case, little or no coordination
among partitions is required. In contrast, concurrent computing might well have tight interdependencies, in the form of contended locks, transactions, or other synchronization mechanisms.
> This of course begs the question of why such a distinction matters, which brings us to the second perspective,
that of the underlying scheduler. Schedulers come in a
wide range of complexities and capabilities, and as a rough
rule of thumb, the more tightly and irregularly a set oparallel processes communicate, the higher the level of sophistication required from the scheduler. As such, parallel
computing’s avoidance of interdependencies means that
parallel-computing programs run well on the least-capable
schedulers. In fact, a pure parallel-computing program
can run successfully after being arbitrarily subdivided and
interleaved onto a uniprocessor. In contrast, concurrent computing programs might well require extreme subtlety
on the part of the scheduler.