I don’t think there is conclusive evidence right now. But what I have heard hypothesized is that books require engaging your mind over a longer period of time on the same narrative thread broken up over numerous sittings. So, you improve your ability for complex, long-term cognition, since you have to commit to memory and recall details about that specific story, not just general ideas related to it.

I think it would also be problematic to exclusively read books personally. Not everything worth reading makes sense in a long format. Technical documentation is also part of my day-to-day.