I'm writing a math book, and after I published the first few chapters, someone offered to buy me a copy of this book. It's a nice gesture if you don't think very hard about it.
Anyway, on to the book. It's got some good points: write for yourself, ruthlessly trim prose, and decide on a consistent tone before getting started. That's it; now you don't need to buy this book as I did.
Zinsser goes on several tirades about the sanctity of language, about how he refuses to use any neologisms, and so too should you. The book suddenly begins to show its age; none of his examples were words I had ever considered to be problematic. Maybe "trek" was once contested, but finds itself safely ensconced in the language today.
The majority of "On Writing Well" is "how to write newspaper articles about topic X." Business, news, artistic reviews, you name it. I briefly skimmed over the science section, thinking it might be helpful in my pursuits. It wasn't. Zinsser takes the viewpoint that science is scary, audiences don't care, and so a writer must do ones best to handhold the reader along. Science reporting at its finest.
In conclusion, I learned a few things, but it wasn't worth my $14. Guess I should have let that guy buy it for me instead.