Spector is a highly respected geneticist. His work on the human genome project and the like is daunting, and is a perfect person to write a book that furthers the understanding of the increasingly complex world of genetics.
The reader can tell that he’s not a typical writer. Many chapters end with the reader being required to deduce the final opinion or result. However, a lot of the research (since its so cutting edge) is not conclusive. It only points to possible theories. Much of the time a final result or opinion is possible. Though I think it would flow better if the author simply stated this.
As for content, he basically sums up why genetics is much more complicated than previously thought. His use of twins in statistically deducing the subtleties of hereditary characteristics is wonderful. A main point is that besides genetic code, there is something called epigenetic coding that turns on or off genes. This allows the genes to either be expressed or not. This is a simplification but is indicative of the main idea.
It’s certainly worth the time. I would read Richard Dawkins’ “Selfish Gene” first though.