What Is Copyright?
Very simply, copyright is the legal protection and registration of authorship in the United States. Copyright is a voluntary process. Technically, copyright is obtained by an author at the moment of creation and application to a tangible form. Therefore, you do not have to have registered your book to have the copyright to it. However, copyright is very advantageous for litigation purposes if you ever needed them, and many authors like to receive a certificate of registration. Once you have applied for copyright and registration is complete, copyright lasts for the full life of the author plus and additional 70 years, generally.
How To Obtain
Obtaining a copyright registration is simple. You can register your work on line through copyright.gov. They also have a wealth of information about other questions you might have.
Consult their FAQ section for many common questions.