Human beings are "the host" for space parasites in this sci-fi novel. Most of the human race has been taken over by the "souls" from space. These invaders destroy human consciousness but leave human memories. That is, until a soul named Wanderer is implanted in Melanie. Her consciousness fights back, leaving her with a kind of split personality. Wanderer has a superior named Seeker. Seeker wants to make sure all humans have been converted and nags Wanderer to hurry up and get on with Melanie's take over.
There is so much more to tell, but I don't want to give away too much. It's really good. The unique aspect of the story is that it's told from the viewpoint of Wanderer, the parasite.
The Host (2008) by Stephenie Meyer is available via Overdrive. You can read the ebook or listen to the audiobook. If you choose to listen, you're in for a treat: narrator Kate Reading does a good job matching voices with personalities.