I recommend you take a look at the way kids hold their phones when doing "serious" fast-paced gaming. The 4 finger claw grip[0] is not very comfortable in the long term. I'm guessing it'll be fine for teenagers, but beyond that I could see it causing issues.

[0] https://www.youtube.com/watch?v=zKrU4yYTGXI&t=4m51s

Books are way less addictive across the board in my experience. You might find a handful of people consuming them to the point of harm, but when you add together all of the addictive activities on a phone/tablet there's no question IMO

Books you aren't repeatedly stabbing with a finger.