I used to be highly skeptical that one could be reading more than one book at a time. But in the last couple of years what I have found works best for me is to have 3-4 non fiction books going simultaneously. I tend to pick books on quite different subjects so I that there is no confusion. Progress on each book is somewhat slower than reading just one but not by as much as I would have thought and my completion rate is way up.
Here is why I think this works. When you have only one book and you are in a bit of a lull on it and have only say 15 minutes of time you are likely to do something else instead of reading (usually something like browsing Twitter). With several books I almost always want to make progress in at least one of them and so that’s the one I will pick up in that moment.
I am sure many people have discovered this before me, but I am thrilled to have found this method! Better late than never.