
The book actually doesn't spend all that much time on this specific episode, which in the end didn't seem all that important to either party. But the episode did present an opportunity to compare and contrast the ages represented by Bach (old school Lutheran, belief in original sin etc.) and Frederick (Enlightenment, rationality, belief in perfectibility of man).
The author gave a brief biography of each protagonist; he concludes Bach was a bit of an individualist within the Lutheran tradition and in fact a precursor to the Romantics.
It was interesting to think of a person like Bach in those days -a village cantor or organist because there wasn't much else of a market for musicianship - recognized for talent, but in a role where followed orders and did music as a craft; his music depicted theological "truths." The notion of an independent artist expressing his or her own vision just didn't exist in that part of the world at that time. Yet Bach did some pretty independent composing, ergo the esteem in which the later romantics held him.
The portrayal of Frederick wasn't very flattering - the author spends a great deal of time explaining how his father was controlling and abusive, and how this appeared to affect Frederick's relationships ever after. He was intent on expanding Prussia, ended up with lots of wars. Apparently relatively progressive as a ruler; also somewhat forgotten until Hitler came along. Not sure why Hitler picked up on Frederick the Great, maybe he was as good a symbol as any that were available; I read that Hitler made the formal declaration of the Third Reich at Frederick's tomb. (I think the "first reich" was the Holy Roman Empire, and the "second reich" was the German empire as it existed after the unification by Bismark in 1870 through the end of WWI.)
The author gave a lot of details about canons and fugues, how they are constructed etc. I'm confident I still don't understand this, and could never pick out the elements - though I certainly like the sound of Bach's work of this nature, or at least the little to which I've been exposed.
Also, the author exults over Bach's B minor Mass and the Passion According to St. Matthew - I would agree that both are great listening. It's also interesting how Mendelssohn did a version of the B minor Mass that was catalytic in bringing Bach to public attention - after 100+ years of Bach having been largely forgotten.