Companies can charge up to $150 to register your copyright on your behalf but you can do it yourself for only $35 and it'll only take half an hour. That's a savings of $115. Now multiply that by all the books you're going to need to register one day.
Why register your copyright? While it's true your work is copyrighted from the time it is created, unless you register your copyright you can't bring a lawsuit against theives for infringement. A related point is that if your book is uploaded to a website and offered for free without your permission the site will often remove it if you send them proof of copyright. In my opinion it's $35 well spent.
For an excellent guide through the entire process go here: A Step-By-Step Guide to U.S. Copyright Registration for Self-Publishers.
Thanks to PG over at The Passive Voice blog for posting a link to this how-to article.
