Sorry - I couldn't remember whether I'd replied or not - but here I am. I tend to have books that are quite substantially different on the go at the same time (1 crime, 1 sci-fi, 1 fantasy or 1 non-fiction) and also different formats - a physical and an audio at the same time, one small physical that I can carry with me, 1 larger physical that isn't really portable - that kind of thing. Think of it like watching different series on TV - you can switch from one to another at the end of an episode or two.