As of now, you can download Aliena by Piers Anthony for free from Amazon (US, ES) courtesy of Premier Digital Publishing. This is the synopsis of the ebook:
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.