This could be anything that you naturally find interesting - books, art, armadillos, roofing etc. Engage with other people that also find this topic interesting.
It'll take several months, but you'll uncover more problems than you'll have time for. And the best part - you now get to pick a problem you care about, and build a solution for it.
Let's have a little fun and pick apart the first topic you listed. Books. What exactly is there still to be done in the realm of books?
Publishing? There are big companies or now you can self-publish. There are many video tutorials, courses, and blogs all dedicated to self-publishing, so even the online education realm of things seems to be taken.
Writing books? Ok sure you can write a book just like you can produce a single piece of any kind of content, so that can't count. Software to assist in writing books? From my quick search, there's Scrivener, and a bunch of other platforms and apps.
Want to write a book without typing? Hmm, there's already a bunch of speech-to-text apps like Dragon.
Buying books? Amazon.
Audio books? Audible.
Collecting books? Hmm ok, may be something in there. Site that lists the top 100 books from prior years? Or a top 10 per genre, per year (historical).
The reality is coming up with an idea is already hard. Coming up with a business is much harder.
Would love an idea in this space.
A) the question asks specifically how to find a "side project" idea, not necessarily a full fledged business all though it could turn into one.
B) The whole point of being involved in a specific community (e.g. fantasy books written by Terry Brooks) is that issues will crop up naturally that you, as a dev, may be in a position to solve. Theorizing on problems in this thread is the exact opposite of my suggestion.
C) The topics were half in jest (see 'armadillos'), and were just meant to kickstart the OP's thinking process.
The process of coming up with an idea shouldn't have to happen in a vacuum. The point it to solve problems you encounter and help solve them.
You're totally right. Just frustrating -- Guess if I want to do something in that space, I need to get more involved in the community, like you said in "B".
This could be anything that you naturally find interesting - books, art, armadillos, roofing etc. Engage with other people that also find this topic interesting.
It'll take several months, but you'll uncover more problems than you'll have time for. And the best part - you now get to pick a problem you care about, and build a solution for it.