I glanced through the book at Barnes and Noble this afternoon and it really looks pretty detailed. Basically, the author has pieced together clinical information, historical data, HIV sequence data and come up with some very plausible ideas about how AIDS initially spread. He talks about how 'amplification' of the virus occurred and postulates that there were four different points at which the virus jumped from apes to humans. He also discusses the role Western countries, as imperial powers, played in spreading the virus.
Nowadays, the M strain is the mist widespread HIV strain. Dr. Pepin talks about why this may be so. Worth a read one day!