You can always write a good book about what you like. It takes time to develop the story, but once you get going, things will get easier. There are a lot of different writing styles out there to learn from. Although you could write about sportsbook promo codes, you are better off writing about something that appeals to a wider audience like binary trading. It's always fun to talk about making money, so by writing a book about winning at an online casino, may also be great topic. Keep you imagination open and good things will come.