Hard to measure the effectiveness of a book. People bought it and read it. Occasionally, someone will deploy it in an argument. It occasionally comes up in forums here.
Because it deals with IQ, it seems as though it might be what the poster was thinking about when they mentioned The Bell Jar. Sylvia Plath’s semi-autobiographical novel doesn’t seem to fit what was put forth.
You are right. Magnus Carlsen was born with a 2800+ rating.
He had rapid improvement - which I explained in my post if you read it.
I'm asking for an example where it's SLOW and steady improvement over a long period of time
I started at 600 in 2018, and am now 1900+