I'm looking to identify a book which is a recent (2010+) paperback of which I've only read the first pages.
It presents the history of earth as the constant struggle of two factions of one alien race (or two races) who use humans as hosts to escape earth. The struggle has gone on for some millennia. One faction is trying to help the humans (and losing); the other fraction's victory will mean the end of humanity.
It's set in modern times. It starts in a city (NY?) in the United States with a fight between two groups of these human-hosted aliens. One host gives his life, so the alien symbiont must find a new host. It does so at the last moment: the host is male and "physically suboptimal"—i.e., a typical nerd.
The aliens need human hosts to survive; without them they seem like globular shadows and will die after some minutes. They physically enhance their hosts and communicate with them telepathically (like a voice in your head).