Ive read a bit of the book and its pretty interesting, But IQ is not a good measurement of intelligence and outdated.It only takes into account measurement of reasoning and problem solving/algorithm.
It doesnt measure, Memory intelligence, Social intelligence, creative intelligence etc. Its also very flawed, search up Rotogenflux Method, basically teaches you how to analyse shapes and shit easier. '
And another important thing that the book doesnt really get into that much is the socio economic positions different races were in at that time.