I somewhat arbitrarily picked up Simple & Direct: A Rhetoric for Writers by Jacques Barzun in a used book store. At the beginning of the book, there are some great quotes. The best one is perhaps the following from C.S. Lewis:
I sometimes think that writing is like driving sheep down a road. If there is any gate to the left or right, the reader will most certainly go into it.
Requires the slightest idea about regular behavior of sheep, but does not have that? Barzun also offers a definition of the perhaps elusive term rhetoric (as many words, it holds so much [to me] that a definition is difficult to put down precisely):
Rhetoric is the craft of setting down words and marks right; or again: Rhetoric shows you how to put words together so that the reader not simply may but must grasp your meaning (p. 2, revised edition; his italics).
Simple and direct, I presume, but Barzun still needed two tries. I think I prefer the latter. Barzun writes sensibly about writing (read well, for example, perhaps the best advice), but feels somewhat outdated in places. For example, he makes a good case for the senselessness of workaholic (derived from alcoholic that comes from al-cohol, worka-holic does thus not work because -holic has no meaning), but I perceive workaholic as established. It is, for example, listed on my dear dictionary.com. (Simple & Direct was published in 1975; the revised edition is from 1985, and some 30-40 years should not matter for a book on rhetoric; Strunk’s Elements of Style, more than 100 years since the first edition, I think, is for example still highly regarded.) Nevertheless, I find Simple & Direct useful and interesting and hope to post more from and on it when time permits (progress is slow, as always).