I wrote a book of fiction (in French) and I self-published it on Amazon (KDP) after some encouragement from someone here on HN; it's just been selected as one of the five finalists of the Storyteller Amazon contest. Sales are slow, though.

I'm trying to get good at EDM but am making little progress. I have watched many tutorials, studied the masters. I am good with tools, and sometimes I come up with 4- or 8-bar loops that aren't that bad, but the songs as a whole are boring. There is something about building expectations, intensity towards the "drop" that I still don't understand.

About a year ago I built a touch-less MIDI controller based on Arduino and cheap HC-SR04 sensors. It's fun to use but I don't know what to do with it. It could be a cheap alternative to a true Theremin but I think the market is too small to pursue.

I'm working on a webapp to teach sight-reading; the market is probably also small but at least there are no moving parts, prototypes, inventory, shipping, etc. There are other apps that do the same thing but I think my version may have an edge. We'll see.