A lot of authors are introverts. They love writing but hate trying to sell the darn thing. Authors moan about no one buying the wonderful book they have written. It's not the book's fault. The author has to get out the door and meet their readers. I have a small 60-page book on how to try to find those readers.
I love getting out the door and finding that arts-and-crafts events are a great place to sell my books. While I'm out, I meet and engage with readers who have book clubs where I can be invited to speak. Other writing groups need speakers, too. You just never know where it will lead you. This month, I have two Arts and Crafts events, and I'm giving a talk to a writing group. I'll also be doing a workshop for another group.
All that is in addition to the social media promotion I put out for my books. My eBooks are with Draft2Digital. They have several promotions during the year that I get involved with. An author needs to stay on top of these and keep promotions going to make those sales.
If no one knows you exist, how can they buy your book?