This book is, as another reviewer put it, definitely worth reading. Singer brings to light several important issues that the left certainly needs to address. However, it is not without its flaws.
For one, Singer misrepresents some of Marx's ideas. Marx clearly *did* have a concept of a fixed human nature, albeit that interacted dialectically with its social surroundings. For more on this view, see Marx's Concept of Man by Erich Fromm and The Dialectical Biologist by Richard Levins and Richard Lewontin.
Also, Singer relies to heavily on the discredited reductionist approach to biology championed by Dawkins and company.
There is no "nature vs. nurture". There is no linear relationship between genotype and phenotype. Almost everything results from nature *and* nurture. Take, for example, even a simple thing like height: we all have different genetic potentials for growth, but only with proper nutrition can those potentials by fully realized. And today, there are even limb lengthening operations, allowing for the phenotype to be further altered -- without genetic manipulation. One can only imagine the multitude of ways in which environment must, then, impact social and psychological development.
Similarly, Singer uncritically accepts Derek Freeman's attack on Margaret Mead. But, as Martin Orans argues convincingly in his Not Even Wrong: Margaret Mead, Derek Freeman, and the Samoans, there's ample reason to doubt Freeman's thesis.
But read it, and make up your own mind.