I find that the way of reading that gives me least eyestrain, is using a phone or a tablet with an OLED (important for "perfect blacks") screen, setting it to some warm color (e.g. orangeish yellow) on black, and dialing the brightness level down to exactly the point where it's readable without straining for a given background light level, but no brighter.
The benefit of this arrangement is that it works in a wide variety of lighting conditions. When I read at night in bed, there's practically no light around at all, and so I dial the brightness all the way down - and read without losing night vision.
Between the ability to do that, and the ability to adjust font size, I find e-books to be light years ahead of anything on paper, in terms of ergonomics.
The benefit of this arrangement is that it works in a wide variety of lighting conditions. When I read at night in bed, there's practically no light around at all, and so I dial the brightness all the way down - and read without losing night vision.
Between the ability to do that, and the ability to adjust font size, I find e-books to be light years ahead of anything on paper, in terms of ergonomics.