As a professional copywriter, I like to believe that my work does not need copyediting. But of course, there are always some improvements that a copyeditor can bring to any text.
As a writer, I have learned a great deal from the book The Copyeditor's Handbook: A Guide for Book Publishing and Corporate Communications by Amy Einsohn. The book's exercises and the explanations that accompany the answers will help you develop your writing skills and understand why copyeditors make certain changes in your work.
Einsohn's book is comprehensive, clear, and very readable. It will likely gain a place next to The Chicago Manual of Style as one of my standard references to check for questions about grammar, punctuation, etc.
As a side note, Chicago has long been my favorite style guide as I find its content is highly applicable to corporate writing. The other popular style guide, The Associated Press Stylebook, is directed primarily to journalists and has less relevant content for my work.
Do you prefer Chicago or AP? Or, do you have another style guide that helps your copywriting?