Here is some clarification from Zinsser himself (ibid.):
"Who am I writing for? It's a fundamental question, and it has a fundamental answer: You're writing for yourself. Don't try to visualize the great mass audience. There is no such audience - every reader is a different person.
This may seem to be a paradox. Earlier I warned that the reader is... impatient... . Now I'm saying you must write for yourself and not be gnawed by worry over whether the reader is tagging along. I'm talking about two different issues. One is craft, the other is attitude. The first is a question of mastering a precise skill. The second is a question of how you use the skill to express your personality.
In terms of craft, there's no excuse for losing readers through sloppy workmanship. ... But on the larger issue of whether the reader likes you, or likes what you are saying or how you are saying it, or agrees with it, or feels an affinity for your sense of humor or your vision of life, don't give him a moment's worry. You are who you are, he is who he is, and either you'll get along or you won't.
N.B: These paragraphs are not contiguous in the original text.
That's not helpful. Say I've got an audience who wouldn't like me if they knew me as my inner circle does, who definitely wouldn't be convinced if I wrote as though I were writing for my own. What would Zinsser do? Give up? Write something else? I know that communicating effectively when you don't personally feel what you're saying tends to fail, well yes, it's hard, but that's precisely what I've got to do!
People want to tell everything instead of telling the best 15 words. They want to learn everything instead of the best 15 words. In this thread, instead post the best 15-words from a book you've read recently (or anything else). It has to stand on its own. It's not a summary, the whole value needs to be contained in those words.
I'll start in the comments below.
(Voted by the Schelling study group as the best exercise of the meeting.)