I just had to deal with this.   I found the offender because I had set up Google alerts for my name, the name of my novel, and some selected text from the novel itself.  If you want to do this for yourself, it’s pretty easy.  Just go here: https://www.google.com/alerts

Steps to take to defend your copyright and some sample cease and desist letters can be found here:  Reporting Copyright Infringement

Also, a good explanation of copyright is available on the SFWA website.