Ratamahatta

Hello. I am reading a book called "The Journey of Man: A Genetic Odyssey" and there is one part that I need some help understanding. It's about Lewontin research and I will just quote from the book: "[...]In the analysis, Lewontin used as his model the new science of biogeography because he thought this was analogous to what he was doing with humans - looking for geographic subdivision in order to define race. In fact, unsure of how to define a "race" objectively, he divided humans largely along geographical lines - Caucasian (western Eurasia), Black African (sub-Saharan Africa), Mongoloids (east Asia), South Asian Aborigines (southern India), Amerinds (Americas), Aceanians and Australian Aborigines. The surprising result he obtained was that the majority of the genetic differences in humans were found within population - around 85 per cent of the total. A further 7 per cent served to differentiate populations within a "race", such as the Greeks from Swedes. Only 8 per cent were found to differentiate between human race. A startling conclusion - and clear evidence that the subspecies classification should be scraped.[...] [...]Lewontin likes to give the example that if a nuclear war were to happen, and only Kikuyu of Kenya (or the Tamils, or the Balinese...) survived, then that group would still have 85 per cent of the genetic variation found in the species as a whole." [...] Now this is very confusing. Can anyone explain to me how genetic difference is measured and what it means? What do all those percentages mean? What does it mean to have 85 per cent genetic difference? Also: you always hear that Humans and Chimps share 96% of genes while in this quote it says that "A further 7 per cent served to differentiate populations within a "race", such as the Greeks from Swedes." Does it mean that Swedes and Greeks share 93% of their genes? It can't be. You can tell that I don't know anything about genetics, but I find the topic very interesting. Please explain to me what it all means. Some book recommendations will also be welcomed. Thanks!