Look intensively before starting work. Make sure that the book you want to write does not exist before. Or reach out to the previous ones and complete the work accordingly to provide the audience with newer content
The book that readers want to buy is the book with the good title, the excellent cover, the striking image on the cover, and certainly the very good opening paragraph.
I already have 2 text books published. If I were to write another one it will be more on how to lead a simple life, the lessons learned by humanity and our future.
If I will get the chance to write a book then I prefer to write a book on Philosophy, Why because only philosophy will give questions where science fails or find to answer.