What is the theme in On Writing Well by William Zinsser?

The craft of writing is the major theme in this book. Writing well is sometimes considered a talent and even an art, but Zinsser thinks of writing as a craft that should be practiced and perfected. Everybody has the ability to write well enough to produce clear prose without useless words or words that carry little, if any, meaning. Still, without clear thinking, clear writing cannot result. The act of writing can help organize thoughts on a subject, with the end result being clarity in both the thinking and writing.