Writers are always looking for ways to promote their books. You need ways to create a buzz about your book. Here's my list of some low cost/no cost ideas.
Newsletter Swap - Find another author who has a newsletter and readers similar to yours. You create a newsletter that she sends out, and then you do the same for her. Each group of readers get introduced to a new writer.
Facebook Takeover - Find an author who will let you take over her Facebook site for a promotion. Promote the event. Then on the day of the takeover, chat with readers and offer small give aways. Usually, these are done for several hours.
Guest Blog - Find other authors/book bloggers who will let you do a guest post or an interview on their site. (Our blog, Pens, Paws, and Claws does author interviews on Wednesdays. I'm currently recruiting for spaces for the summer of 2018. Let me know if you're interested. This blog is about writers and their pets.)
Twitter Party/Chat - Host a Twitter Party or Chat. It's usually better if you can recruit several authors to help. Talk about your books and writing and have small give aways.
Facebook Party - Host a Facebook Party on your author site. Talk about your writing projects, chat with readers, and do some contents/give aways. It's usually better to start a new post for each different topic.
Facebook Hop - This is a scheduled event (usually over several days). When you organize one, you recruit authors who will participate and give away a prize. The organizer lines up all the participants, provides the promotional information, and the links. Each stop on the hop is linked to the next one. (All links have to work, or the hop stops.) It takes a little bit of time and energy (and patience) to organize one, but it's a way for all the authors to gain new readers and followers. (I have a blog post on how to host one if you're interested.)
What else would you add to my list? Best wishes with your writing and book promotions!