Even if you have your own strong opinions about a topic, make a research before writing your book. Cite previous research (if it is the case) and express your own views on them. Do you agree with them or not? Your own point of view regarding the previous research can be of huge importance, especially if your book is a scientific one.
If you write a non-fiction book, you need to know that the way you transmit the message is crucial. Do you write complicated or long sentences, without any coherence? Then you should probably need a stylist who can help you with the writing. A clear and simple sentence is essential when you write a message.
A clear and structured book reveals the author's mind. If you are a logical person, perhaps you don't need this advice. The order of the chapters should follow the logical flow of dependencies: what is the book about, a short presentation of the previous research, the new approach that you propose, analyzes, tests and conclusions.
Selling a book is a complex process. You need advertising and a lot of visibility. How to gain that? Choose the best title for your book! If the title is attractive, readers may come without realizing it! A subtitle could give additional and valuable information about the content of the book.