Interesting –

The bottom line is that your book is automatically copyrighted, but if you have nothing better to do, you can register it. It only costs $ 35. However, we doubt that you’ll have the time or resources to sue for statutory damages. And even if you win, we doubt that you’ll collect the damages. The entities that have money can probably out-litigate you, and they are unlikely to purposely violate your copyright. Shawn and I have never registered any of our past books. When we have encountered file-sharing sites distributing illegal copies of our books, we’ve told our publishers, and their response has been “There’s nothing we can really do about it.”

Kawasaki, Guy; Welch, Shawn (2012-11-30). APE: Author, Publisher, Entrepreneur—How to Publish a Book (Kindle Locations 3307-3313). Nononina Press. Kindle Edition.