I like to take notes, it helps me focus.
Back in university, during lectures I would always take notes, in part to avoid falling asleep. Pen and paper, a separate notebook for each course. I rarely actually studied from them, though. But I did use them to look up things from previous years, and I sometimes still do.
When reading books, I like to "outline" the book. How detailed the outline is depends on the book. Places that feel more important are also annotated with page numbers to help finding them again. If I read on the computer I take notes on the computer, otherwise I use pen and paper. Once I finish, I write a short "review" about the book, what it was about and my impression of it, but nothing too detailed. There were books that I really liked where I also wrote summaries of the chapters or concepts, etc., but that's usually too much work and I am lazy.