Harris's book must have been really ancient history by the time you got to it! I read it in 1957 at the age of 17 and spent a few months as a "jazz purist". His extraordinary message seemed to be that if a band hadn't got a banjo in it, it was no longer jazz and that Ellington ceased to play jazz when he added saxophones in the late 20s!!! Fortunately, Bird, Diz, Monk and Miles blasted me out of that way of thinking when I was 18.
I seem to remember that dear old Rex also asserted that Hawk would have been a great jazz musician if he had played an instrument more suited to jazz, like the clarinet.