Assuming you are talking about a self-published book, there are a number of ways you can promote your book in 2021. Many of these methods are free or relatively low cost, and can be done yourself with some planning and effort.
Here are some ideas to get you started:
Create a website or blog for your book. This is a great way to centralize information about your book and make it easy for people to learn more and buy the book. Be sure to include buy links on your site.
Use social media to promote your book. Create accounts on Twitter, Facebook, and other social media sites, and use them to share information about your book. Use hashtags, post links, and run giveaways to attract attention.
Get involved in online book communities. There are many online communities for book lovers, and getting involved in these can help promote your book to a wider audience. Share your book in forums, on book club websites, and on social media groups devoted to reading.
Reach out to book bloggers and reviewers. Most bloggers and reviewers are happy to receive pitch emails from authors, so reach out and introduce yourself. Many will be happy to review your book or feature it on their site.
Plan some offline promotion. In-person events may not be possible right now due to the pandemic, but there are still some things you can do. Organize a virtual book tour, set up a booth at a local fair or market, or host a contest or giveaway.
With some effort and planning, you can promote your book effectively in 2021. These are just a few ideas to get you started; for more detailed information and advice, check out this article on book marketing for self-published authors.