Reddit reviews The Journey of Man: A Genetic Odyssey

There are not three major groups of humans. This information is outdated; it does not take modern genetics into account.

Based on recent genetic research, if we were to split the world population into a handful of major lineages, there would probably be just two still living today: the San people and everyone else (Asians, Aborigines, Caucasians, Native Americans, the vast majority of Africans, Polynesians, etc.). (If you want to learn more about this, The Journey of Man by Spencer Wells is a good start.)

But within these two groups, there are still a great many only-distantly related lineages. Africans in particular are an exceptionally diverse group of people—which makes sense, because Africa has been populated for the longest period of time by far, which means there have been many more generations of people there diversifying. There are African lineages that are more closely related to Asians and Caucasians than they are to other African lineages. This makes sense if you think about it; by the time the ancestors of Asians and Caucasians crossed into the new continents, Africa would have already been well populated (perhaps overpopulated, driving these migrations), and only a few African families would have spawned the Asians/Caucasians, not all of them.

As you can see, the Fula people have very different facial features and skull shapes from the Mongo people, who are distinct from the Igbo people, and so on. Don't be fooled by the similar skin colors; dark skin color is an adaptation to the sunny conditions in Africa and does not imply genetic relatedness.

And just because people make this error a lot, I want to point out that African Americans do not represent Africans, and any study that attempts to make this connection is flawed. African Americans are derived almost entirely from a small proportion of the African population—and from a relatively small corner of that vast continent—plus Caucasians to a lesser extent (white slave owners often fathered children with their female slaves, and plenty of intermixing has occurred since then). Africa is drastically more genetically diverse than that.

(Edited to fix link)

Not specifically about the Caucasian migration, but The Journey of Man: A Genetic Odyssey is an interesting read on human migration in general. Or you could check out Journey of Man video