Check out this article: How to make money as a politician? Write a book
I’ve made money from my books. What I like about this article is that it tells you how much each of these people made from their books. The money isn’t overwhelmingly impressive, but you can see how they make money, and on what topics. I’m not suggesting you need to be a politician in order to make money as an author… you can do better than these guys have. But this article gives you an idea of what you might be able to expect, financially.
Sometimes, you just gotta get that book out of your mind, even if it has nothing to do with money. But heck, if the money is there, why not work towards it?
Here’s something else: you might not make much money from your book(s), but the money might come from speaking, consulting, etc.