Around fifteen years ago, some books came out suggesting enhanced abilities associated with reading processes (Proust and the Squid) and the effects on thinking from types of text read, such as online/social media (The Shallows).
I’d guess there’s a correlation between how many books a person has read in a lifetime and intelligence, but one of the factors there will be duration of education and investment in the educational process (many other factors, too). It would be tough to be sure about the relationship to people’s habits in adulthood (on reading for pleasure or personal edification), for reasons touched on in other posts. I believe proponents of reading making you smarter suggest that some features of memory, processing, concentration, pattern recognition, etc. can develop regardless of content (as for the poster who sees trash on the bestseller lists). But it nevertheless holds that more challenging material creates greater improvement. I liken it to running. Any running is good; well constructed training is better.
i mean, I suppose I’d have had a chance to make myself smarter by taking more time to understand Feynman’s 3 book lectures on physics and could have abandoned the Gleick biography of him without losing a thing.
There’s the obvious, though: Some nonreaders are exceptionally intelligent, and some voracious readers aren’t especially sharp.