"There is a wide consensus that the racial categories that are common in everyday usage are socially constructed, and that racial groups cannot be biologically defined" - wikipedia
There's simply no scientific basis or definition of "race" as Nicolas Wade uses the term. People in the bookstore will presume he's talking about melanin. Three hundred years ago Spaniards were considered a different "race" than Anglo Saxons or Greeks. To suggest that the "learning gene" is somehow incompatible or cannot be passed on in combination with a certain skin color / melanin gene seems obnoxious if that's not what the data show. Most "races" as defined by book-buying public are hetero-genetic, it may indeed seem to some either reckless or cynical of Wade to work "melanin and intelligence" into the book title. If I inherit dark melanin from my father and intelligence from my mother, I'd be more than just "politically correct" to be pissed off at Wade for implying that my dad's skin color negates mom's smarts.
It is controversial enough that tendency for intelligence can be inherited. The fact that skin color can also be inherited is true. Height can also be inherited, and hairlines. To insinuate, through the title of the book, that "race" is more correlated than height/hairline may be true (or not, I don't know), but if it's not determinative of intelligence, it doesn't belong it the title. Some people objecting may indeed object out of so-called "political correctness", but unless the skin color gene is somehow genetically incompatible with intelligence, it's just creating a non-useful stereotype.
Since there is no link to the letter of objection, those /.ers whining about "political correctness" are merely guessing at the motive of behind the letter of objection. My personal guess is, "don't take years of our scientific data and pick two traits - melanin and learning - and imply that those two traits, out of thousands of other traits, are tied together in some way just to promote your book sales."