I have some little rules around this. For example, I can't read two novels at the same time, but the minute I finish one I'm immediately on the lookout for what to read next, so I always have some fiction in progress. I'm reading The Lifeboat by Charlotte Rogan at the moment.
Whichever novel I'm reading, it is usually accompanied by a biography or autobiography... currently in the midst of David Mitchell's Back Story... it's pleasant and clever but disappointingly slight.
In addition, I'll often have a business or self-help kind of book on the go. This week I tore through Clarity by Jamie Smart and I highly recommend it if you ever feel somewhat overloaded, or stressed, or simply can't think straight. He brings home some simple, universal truths which literally make you think... in the right way.
I'll also have either a travel or a history book in progress either to widen my horizons or to offer some perspective on current events. Andrew Martin's Underground Overground is currently ticking both boxes for me.
Then I'll often be reading a graphic novel or series of comics. The non-fictional aspect seems to clash with my difficulty in focusing on two novels at once, but somehow, I seem to be able to manage comics alongside novels. I'm currently enjoying Saga by Brian K Vaughan and Fiona Staples via the ComiXology app and really recommend it.
Lastly, I'm often in Flipboard, Pocket and any number of social apps catching up on news, etc.
This approach might sound overcomplicated but it seems to work for me.
What works for you?