I built a free audiobook search engine a couple of months back to scratch my own itch. It has been one of my most successful side projects to date. I got around 9k unique visitors in the span of a month after I got to the front page of HN.
Yet now I don't know how to make it profitable. This is something I am interested in building. I simply don't see the potential in affiliate links.
Check the month of February: https://cloud.umami.is/share/V1zn2LVYcBKhQzT7/booksearch.party