Here is a secret. I never read one book at a time. That probably is not all that much of a secret, but I know it is a reading style that not everyone agrees with. However, it works for me.
What I read varies. Sometimes I will be reading a non fiction book and a fiction book. Sometimes I will be reading from one of my lists while reading a book not on my lists. It really varies based on my mood. I mean reading is about fun an enjoyment. It is about distressing and discovering other worlds. If reading becomes stressful then it is time to take a step back. . . but I suppose that is another post.
Currently I am tending to read books based upon their format. I have an audiobook, a paper book, and a kindle book all in the works right now. When I finish one I tend to start another one of the same format and keep going with the other books I have.