I spend a lot of my free time studying computer science. For about a year and half, I spent about 20 hours/week. Now I am down to only about 10 or 12.
Historically, I typically studied only one item at a time, whereas I had a friend who would do two or three. I believe his approach to be better and have adopted it.
When you study a single technical book, all items are not equal. The earlier items provide a required base for the later items; you have to fully understand and recall them in addition to the later material you are studying. What helps with that fuller comprehension? Removal and time.
If I read part A of a book and wait until the next day to read part B, my subconscious mind has a chance to process that material and master it. Thus, coming back the next day makes reading and comprehending part B all that much easier. It the same science that allows me to suddenly make connections on problems I have been working on when taking a shower.
Sometimes you may be aware of that processing. For example, some nights I may recall having intense problem solving dreams around something I am studying as my brain forms new neural networks. This also works for new video games.
Now, I am studying about five different subjects simultaneously. Not only is the process less taxing and stressful, but I am mastering it more effectively.