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.
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
