Olson provides one of the most accessible accounts of population genetics, the fascinating field pioneered in the trenches of WWI and later developed into a full-blown science through the efforts of L.L.Cavalli-Sforza and many others. The book is divided into sections corresponding to parts of the world, though this is only a very sketchy division (since the human race is intrinsically interconnected, one can't really speak of Africa without touching upon the rest of the world).
Olson is not a scientist, he's a science journalist, but he's done his research well, and he certainly delivers his message. There are some concepts that are not very clear in his account; for example, the we all have a MRCA - most recent common ancestor - and he, or she, or both, could have lived much more recently than we think, is pretty clear; but Olson also claims that we are all descended from Julius Caesar, or Xerxes - if that person's life did not eke out, today there isn't a single individual on the planet who is not descended from him or her. Olson doesn't really explain it, nor posits a date ante quam everyone's everyone's granddaddy.
Olson is a proponent of the no-race idea, which I myself tend to support because I find it attractive. However, serious objections were raised to such treatment of human diversity, and the idea itself was termed Lewontin's fallacy (by the name of the American geneticist who came up with it). Olson does not address the issue, nor, indeed, lists any recent, scientific objections. It's easy to dispel 18-century notions.
The book searches for the 'oldest existing human populations', the Khoi-san or bushmen of Africa; examines the effects of long-term inbreeding in Samaritans; explores the cohabitation of anatomically modern humans (that's us) and Neandertals in the Near East. One of the chapters is devoted to the apparent racial and ethnic harmony of Hawaii (not without its problems, of course); I never thought Hawaii were so special.
With a couple of caveats, this is probably the best entry-level book on population genetics.