Copyright and Mandatory Deposit with The Library of Congress
You own the copyright in your work the moment you create it, without any registration or other formalities. Then when you print your book, you identify the copyright using the © symbol with the year of copyright and the copyright owner, like this:
© 2007 Margo Lane.
Finally, after your books have been printed, you register your copyright with the Library of Congress to establish a public record of your copyright ownership. For more details, see the web site for the Library of Congress here.
There is a legal requirement that you deposit 2 copies of your published work with the Library of Congress within 3 months of publication. You can combine this Mandatory Deposit submission with registration of your copyright.
For all the information you need about copyrights, visit the Library of Congress web site here
Library of Congress Cataloging in Publication and Pre-assigned Control Numbers
The Library of Congress has two programs for assigning you numbers before your books are printed. The Cataloging in Publication (CIP) actually catalogs the book, and it is intended for larger publishers. The Pre-assigned Control Numbers (PCN) program is open for all US publishers. There's all the detail you could want on the Library of Congress web site at the following two URLs:
For information on CIP and the process of getting Library of Congress numbers, click here
For information on Pre-assigned Control Numbers, click here.
There is no requirement that you have a CIP, and this is available only to large publishers. There is no requirement that you have a PCN, and frankly I don't see any advantage to getting one, except it may add class to your copyright page.