The best advice I ever received around passive income / side hustles was to do it around something you know about.
For example, I love football and know heaps about it. Literally spend my life watching or playing it. My friends all watch it and I spend time around people talking about it. So I had loads of experience of what people wanted/needed.
So I started a side hustle selling football related t-shirts because I knew what to sell and what people wanted. It’s not massively successful but it does okay and I enjoy it because I know about it. I feel confident.
Don’t start a side project that you don’t know anything about as you’ll grow to dislike it when the times get tough. Your lack of knowledge around the topic will lead to you ultimately dropping it off and not enjoying it.
I think this advice can apply to pretty much anything in life. If you are going to write a book then you should write it about something that you know!