I had to do copyright registration myself this time because the printer for The Woman Who Values Herself did not include copyrighting among its services. I've paid others to do this for me, because when I first did it in 2005, filling out forms and sending them through the mail, I messed up and the process took 18 months. But now you can apply for copyright for your book, published or unpublished, online, today through www.copyright.gov's eCO (e-copyright) system and it costs only $35 compared to $65 if you apply on paper. The online application requires a little patience for clicking Help and FAQ buttons on the less-than-intuitive interface (which advises you that its maximum file size for the "typical 56kbps modem is 11.3MB," and that its system was built for IE and Netscape browsers. Netscape?!? Gesundheit!). Internet Explorer or Firefox browsers are okay, and I think you can load bigger files now.
You can then upload your text if it's in the right format, which is most anything except .epub, which you'll have to convert to .zip. If your book is already printed, apply electronically for copyright and pay the $35 fee online, and then print out a shipping slip to mail along with two copies of the book to the Library of Congress. I just love the idea of my books in the U.S. Library of Congress. The site warns you that your package will be x-rayed for security reasons. I love thinking that my envelope with two books in it is so important that it scares them up on Capitol Hill.
The whole point of formal registration is to establish yourself as the copyright owner should a dispute arise. Probably one won't. But never say never. Registering your book within three months of publication gives you extra rights in case of litigation.
Copyright.gov is a great site for answering any and all copyright questions about texts, music, video, or any other sort of intellectual/artistic creations.