This is the most controversial book in the psychology field! DO NOT read this if you want accurate information on Intelligence. They only used a white sample for their study, and so many psychologists have disproved most of their claims, simply by including more demographics in the data. Please do not make any political choices using this, there are much better books out there!
For a good idea on what H&M wrote and the psych community's response to the book, read "The Science and Nonscience of Psychologists’ Responses to The Bell Curve" by Clayton P. Alderfer (2003).