As a author I think it is very important to diversify your income. For any entrepreneur its important to have more than one stream of income. Your main source of income should be your books but there is also other ways you can make money as a author and its important to take advantage of those opportunities. In my opinion there are two real side hustles for author; create resources for authors and create resources for readers.
Resources for authors. So creating resources for authors is doing what I am with this blog. It’s offering tips and advice and maybe even courses to help other authors on their writing journey. You could become a editor or beta reader. This is something I feel very passionate about so for me it was a no-brainer. I genuinely want to help people write their books and market it to readers. The way you create income from this could be through ad revenue on a blog or youtube. If you create courses you can gain income for that. Your goal should really be to create value. And once you have something valuable to give authors the profits will follow.
Resources for readers. So by resources for readers I mean becoming a part of the reader community. Doing things like book reviews, sharing books you are reading. You could create a book review blog where you post reviews and get ad revenue. You could make a youtube channel and get ad revenue from that. If you have a large fan base you can create merchandise for your book. I even know authors who have started book box businesses where readers sign up for a monthly membership and every month they get a box that has bookish items and a new book or two.
In summary find how you want to add value to the community and the money will follow.