I'm in the process of reading it (page 100 out of 608). It's about these like invader species who come to earth and take over humans body. So the the human body is the host. So they stick 'souls' in the human body and normally the soul just takes over. But in this specific case the human is still there. The human girl, Melanie starts to talk to the 'soul' and things. The soul's job is to like find out where other non-changed humans are like Melanie's love Jared.
That's all I've read so far. It's actually pretty good, which I didn't expect. It makes you think and makes you wanting to read more.