The message of On Writing Well is that anyone can learn to write well. The book is divided into three sections: "Principles," "Methods," and "Forms." In the "Principles" section, Williams covers the basic mechanics of writing, such as grammar, syntax, and style. In the "Methods" section, Williams provides specific advice on how to overcome common writing problems. In the "Forms" section, Williams discusses the different types of writing, such as expository, persuasive, and narrative.