What to read after Understanding Intelligence?

"Gentleman scientist Sir Francis Galton followed much of Spencer's drift. He was, in addition, very practically minded. In possession of a fortune (inherited) Galton was able to indulge many interests. Travels in Africa in the 1850s had already convinced him of the mental inferiority of its natives. He had read Charles Darwin's theory, conversed 19 with Spencer, and became convinced that there is something he called "natural ability". It varies substantially among people, he argued, just like height and weight, and is distributed like the bell-shaped curve. Later he observed that members of the British establishment were often related to each other. That convinced him that differences in intelligence must lie in biological inheritance, which also implied that society could be improved through eugenics, or selective breeding programs. However, he realized, that would need some measure, "for the indications of superior strains or races, and in so favouring them that their progeny shall outnumber and gradually replace that of the old one""--