[agentultra]

That's literally what I do though... I keep journals of what I read. I work on problems if the book comes with any problem sets. I summarize key topics in my own words.

If the book doesn't have problem sets, per se, I come up with ways of testing myself that I understood the material usually through a small side project or by revisiting something I did before and trying to rework it using the new knowledge I'm trying to gain.

I also do this for fiction. I keep journals of all the books I've read, what I was thinking about at the time, my impressions while reading it, interesting passages that stood out to me, etc. I re-read books and I write new journal entries for the second (or third, etc) reading: context and history often change how I think about, react to, or enjoy a story.

Thinking to me starts with writing.