According to the book genetics seems to be the single biggest factor for a ton of things in our psychology. Including various measures of intelligence and achievement. As a layman the evidence presented seemed quite convincing. The book mostly refers to big studies with thousands of twins. To separate nature and nurture studies can do things like tracking twins given up shortly after birth into two different foster families who never had any contact to their birth parents later (correlated nature), track the development twins in the same family (correlated nurture) and so on.
According to Plomin the effects are huge (especially compared to what usually would be a significant result in a psychology study looking at nurture) and the controversial nature of such findings made the field extremely rigorous to the point where it is hard to imagine these results turning out to be wrong. Apparently it has all been replicated quite a bit by now.
To be honest I found the book quite shocking because it runs counter to so much that is "common knowledge" and "common sense". It basically mostly discards influenceable "nurture" as a defining influence at the population level. Once certain basics are met, relevant environmental influences are mostly too random to control and much of the rest is indirectly caused by "nature". "Nature" basically creates its preferred environment (called "nature of nurture" in the book).
As I am not in the field my ability to verify what the book says is limited. As far as I can tell Plomin is a very well regarded Psychologist and mostly know for his involvement in twin studies. If anyone in the field has better insight on how to evaluate what the book says I would be very interested.