How can I retain the most/study the most effectively using a textbook?
What I'm currently doing is writing things I think are important in a separate notebook, not as a reference further on but rather as a means of augmenting my retention. I used to also highlight/underline, but I've come to find that that's a waste of time and is pointless, as marking something I already know won't do anything, while noting it down might.

However, I feel like writing things down as much as I do is terribly inefficient (waste of time, could go through a textbook way faster without) and doesn't actually accomplish better retention, as once I've read something it's usually in my head (can't always recall it immediately though). And my notes are pretty redundant, so I probably just suck at it too (OCD, want to write down everything).

What do? How do you study? General studying tips welcome too (especially Math and Comp Sci).