Book piracy is on the rise...as if we didn't have enough to contend with from Amazon and such, right?
As the author of your book and the copyright holder, you are obliged to send the offending site who's giving away your book a DMCA Takedown Notice. It does not matter if you're self publishing or with a publisher, you are the author and the ultimate copyright holder so this is your responsibility.
Where are pirated books coming from? Well, mostly from Amazon! If you look at the page source for books, you'll see book covers are coming out of Amazon's images files, and the books are filed by the book's ASIN, which is ONLY available from Amazon.
Perhaps there are come consolations to pirated books:
1) Most pirated books which are being given away almost always have data mining viruses embedded in the book files. No one gains anything from giving away your book. But by embedding a virus to infiltrate the user's system, they can sell that information to the next group upstream. What are they looking for? Credit card and banking details...it's all about the money!
2) Maybe sites which give away your book are actually only giving away the same info that Amazon does...the first 20% of the book. Readers don't realize this until after they've downloaded the book and read to the cut off point. Meanwhile, those viruses have embedded themselves into the user's computer and are sending your personal details to the end user.
3) Sites which are selling your book without permission is another problem.
In either of these cases, as the author, you need to make contact with the DMCA Takedown Notice and get the ball rolling. Unfortunately, this is not a case of them proving they have the right to publish. It's your responsibility for proving they don't.
Welcome to the 21st century!
And authors, if you're sending unprotected ARCs for review, be SURE to keep good records on who you've sent your work to, and get a turn around date. Ask where they post their reviews, and check their previous book reviews and hosts to verify the reviewer is who they say they are. And if you're giving away copies of your books, give them as gifts through the 'give as gift' option at the booksellers rather than sending ecopies.
And familiarize yourself with the DMCA Takedown Notice.