"On Writing Well" is a guide by William Zinsser that focuses on the principles of good writing. It emphasizes clarity, simplicity, and the importance of finding one's unique voice. Zinsser provides practical advice for various forms of writing, including nonfiction, and encourages writers to revise their work thoroughly. The book is structured around key concepts such as the importance of audience awareness and the value of brevity. Zinsser also shares personal anecdotes and examples to illustrate his points, making it a valuable resource for both novice and experienced writers seeking to improve their craft.