First, make sure it’s necessary. If you have kept files on your computer with your book drafts and the final version, there will be a date next to them. Same with blog posts that are time and date stamped. In this case, if a dispute arises, you will have proof of ownership.
If, for peace of mind, you still want to register copyright for your book, then you’ll need to go to http://copyright.gov/eco/, fill out some forms, pay the copyright fee and you’re all set.