When reading non-fiction I treat it as if i'm sitting in a college class and the author is giving me a lecture on something, except I take a lot of pauses to digest, otherwise I find that I just keep going without integrating much. I have to stop every ten pages or so to grab some water or change music or w/e. It's only for a couple of minutes but it helps me a lot to think about what i just read and kind of keep it fresh in my head, and eventually it just registers haha (Note that most non-fiction I read is either research or history books)