I like books on writing. I even have a separate shelf for them. My collection of books on writing has been growing steadily even though I don't write at all.
Most books on writing, even when posing as style manuals, represent a way of life, a philosophy on how a soul should read or be read. Reading On Writing Well was more like reading a guide to living than reading a guide to writing nonfiction -- Zinsser does not stop to emphasize how important it is to be yourself, how important it is to see the human, and how important it is to be personal.
The middle of the book is about writing different types of nonfiction: travel writing, criticism, humour, sports.. and Zinssers gives a generous amount of names and examples. I didn't enjoy these more specifically instructive chapters as much as the more general opening and closing chapters, but I can see how they would be very useful for people who do write.