When you create your own content, that material has intellectual property (IP) rights attached to it. You, as the creator and owner of these rights, can exploit them or license them as you see fit and earn income from that.
Therefore, if you’re an indie author or indie creator of any sort, you should become familiar with your IP rights and explore the various ways to generate income streams from them.
In the case of books, this can include creating various formats, such as print (hardcover, paperback, large print, etc.), digital (i.e., Kindle .mobi, everyone else .epub, PDF), audiobook, and different adaptations (film, podcast, radio, graphic novel, etc.).