For some books colour is essential.
This is the piece of string question.
If you have a good idea but don’t feel you have the time or ability to write your book there are plenty of ghostwriters who will do the job for you, at a price.
It is always useful to get someone else’s opinion before you publish but make sure the person you ask knows what they are talking about.
I write business books and to research a new subject I would have to buy or borrow as many books as I can on the same subject and pore over them.
If you want to sell the book, however, you must decide if there is a market for it in advance.
The cover is the shopfront for your book. If you get that wrong you will not sell your book at all.
Most of the available word software will have the necessary requirements for writing a book.
In the distant past your book would be typeset by a professional. If you have a publisher they will still have a professional doing this work for you.
There is no easy answer to this question. If there was, we would all be selling thousands of books. If you want to sell the book, however, you must decide if there is a market for it in advance.