The LCCN or PCN is a number that is assigned to your book and appears on the copyright page. This is the reference number that is found on your book’s Library Catalog Card for location. This number is important if you are looking to sell your books to libraries and is best to apply for before your book is published so it can go on the copyright page.
The Library of Congress has a cataloging system… PCN and LCCN. There is an application to participate you’ll need to apply here http://www.loc.gov/publish/pcn/ You might want to take a look at this now so by the time your book is done you’ll have your numbers and be ready to go.
“A Library of Congress catalog control number is a unique identification number that the Library of Congress assigns to the catalog record created for each book in its cataloged collections. Librarians use it to locate a specific Library of Congress catalog record in the national databases and to order catalog cards from the Library of Congress or from commercial suppliers. The Library of Congress assigns this number while the book is being cataloged. Under certain circumstances, however, a control number can be assigned before the book is published through the Preassigned Control Number Program.”
If you need to copyright protect the material here are the links