On the title page of books published traditionally, you will see something like this-
Library of Congress Control Number: 2012932337
Hogan, Cindy M.
Protected/Cindy M. Hogan.-1st ed.
Summary: When terrorists find Christy, she must enter the Witness Protection Program and become someone totally new while abandoning everything she has ever known.
You want this in your book, too and you can!
A PCN is a preassigned control number for print books only. The book must also be printed in the US and you must get the number before it is printed for the first time.
What is it exactly?
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.
Why do you want one?
The purpose of the Preassigned Control Number (PCN) program is to enable the Library of Congress to assign control numbers in advance of publication to those titles that may be added to the Library's collections. The publisher prints the control number in the book and thereby facilitates cataloging and other book processing activities. The PCN links the book to any record which the Library of Congress, other libraries, bibliographic utilitites, or book vendors may create.