Quite recently, I came across the Zettelkasten system, an analog note taking system developed by Niklas Luhmann. A précis of the system, is that if you make the right sort of references of what you read, and the right sort of notes, you can easily process thoughts from reading into books.
I’ve also been reading a bit more about bullet journaling:
We can often have ideas, whose time or purpose isn’t clear yet. That doesn’t make them any less valuable. We can use our Index to remain aware of our ideas, and perceive them in a way our mind struggles to. The mind can be a jumble of ideas, because they come to us randomly. Our thoughts and ideas are separated by time and context. The Index can remove the distance between ideas by re-contextualizing them as clear list. Just like we use migration to thread the needle between thoughts to from ideas, we can use the Index to thread ideas together into creating the work.Rethinking Creativity, web page by Ryder Carroll
The same theme, slightly different process: keep a track of random ideas, let them marinade for a while, and they will coalesce into something new.
The difficult bit, of course, is developing the system where they can connect and marinade.