The book's introduction is excerpted online. It's accessible reading, starting with some memorable anecdotes that motivated him to write the book.
He goes on to discuss the prevalence of mania and hypomania, both in the general population and in families of entrepreneurs. He draws a nice correlation between the incidence of bipolar disorder in various countries, and their immigration rates. He acknowledges the difficulty of measuring hypomania in a population, but provides more statistical proxies than had occurred to me, including relocation and job-switching.
Another nice touch is when Gartner lets Alexis de Tocqueville make his point for him, quoting the Frenchman's impressions of early Americans. De Tocqueville, it turns out, might as well have been describing hypomanics -- you can compare his remarks to the DSM-IV definition, also covered on Gartner's introduction.