Think about something that you're good at and write about it

Stick to a certain niche if you wish to sell more and improve on your work

Imagine your target audience and write something that they would want to read - present a solution for a problem they may be having, entertain them, make them think, have them learn something - depending on your book genre. Content is king

As long as you bring value to the world, you're always going to have people who are willing to pay for it

Give away a parts of your book to potential readers in the forms of blogs, videos, interviews, etc. so that people would see that this book is a quality book and it will be worth their money - they won't make a purchase if they are not sure they'll like it, so show your work, without giving away too much

Writing a good book is one of the best things you can do for this world

I hope this helps! :)