Piers Anthony 
Aliena 

What’s a girl to do when her mind is not her own? A provocative novel of speculative fiction from the New York Timesbestselling author. Aliena is a nice person who turns out to be an alien in a human host. She just has things to learn about the human form and society. When her body starts immune-rejecting the alien brain, and the brain has to be replaced by another alien brain, maintaining the semblance of marriage and family becomes a challenge for both Aliena and the man who loves her.