The Host

The human race has been taken over by small parasitic aliens called "Souls". They travel to planets inserting themselves into a host body of that planet's dominant species while suppressing the host's consciousness. They access the host's memories, and occupied hosts are identifiable by silver rings in the hosts' eyes. A human on the run, Melanie Stryder, is captured and infused with a soul called "Wanderer." Wanderer is asked by Seeker to access Melanie's memories in order to discover the location of a pocket of unassimilated humans. Melanie's consciousness, however, has not been completely eliminated, and it struggles to regain control of her body. Melanie and Wanderer carry out an internal conversation and debate with each other, forming a friendship. Read more on Wikipedia.