This is the only book that I've read which made reading and writing non-fiction enjoyable. If it was not for this book, I could not have written my first book nearly as well.
It is said that brevity is the mark of wit. William Zinsser exemplifies this perfectly in this book, namely that most adverbs are unnecessary. In one chapter, he has three complete sentences in a row with only two words each, the subject of each understood to be "you." Be content. Be annoyed. Be angry. How can you be slightly content? Slightly angry?
The key point of "On Writing Well" is that every word you write which does not do useful work must be removed, thereby improving the efficiency and clarity of your subject. Why say, "at the present time" when you can say "now." Why say, "the fact that" when you can say "because." Do not be afraid to say, "It is raining" instead of, "At the present time we are experiencing precipitation." I love advice like that! There are many other jewels of knowledge which turn a rough sentence into a polished jem.
The best chapter in this book is "Bits & Pieces," which does an excellent job of giving the reasoning behind good and bad writing. The chapters on travel, technology and criticism are especially helpful, and the author, in the process, takes away the doubt that can crush an otherwise good writer. Mr. Zinsser does not try to sound profound, but rather does a professional job of "instructing" the reader as a student and collegue. He writes from the presumption that you only have a minimal understanding of English.
This book is like having a personal tutor every time you open it. I have read it over at least five times, and I occasionally find what the author was writing between the lines. It is not, by any means, to be compared with a boring textbook. "On Writing Well" was written by a man who truly loves his work, and it shows.