The written word looms over William Zinsser. The many hundreds of books in his Upper East Side apartment stand at attention, as if awaiting instruction from this slight man in a baseball cap and sunglasses who, for a half-century, has coached others on how to write.

In newsrooms, publishing houses and wherever the labor centers on honing sentences and paragraphs, you are almost certain to find among the reference works a classic guide to nonfiction writing called “On Writing Well,” by Mr. Zinsser. Sometimes all you have to say is: Hand me the Zinsser.

“Clutter is the disease of American writing,” he declared in one passage that tends to haunt anyone daring to write about Mr. Zinsser. “We are a society strangling in unnecessary words, circular constructions, pompous frills and meaningless jargon.”

The book, first published in 1976, grew out of a writing course that Mr. Zinsser taught for several years at Yale University. And he is still teaching at 90, holding one-on-one counseling sessions for accomplished and aspiring writers at a round wooden table close to those bookshelves. The only difference is that he can no longer see.