Don’t become a “book-spammer.” Instead, establish personal, helpful, mutually rewarding relationships with your readers.
Many successful self-publishing authors are convinced that paid ads are usually a waste of money. I agree. (Though there are exceptions; see below.)
I also think that many self-publishing authors are inept when it comes to using social media. They send out constant email blasts and Tweets about their books, becoming pests. That’s because they approach social relationships as “takers” rather than “traders.”
A trader knows that if you want to get something from someone, you have to give back something in return.