If you are going to market your book to the approximately twenty thousand libraries that subscribe to the Library of Congress catalog service then you need to have a Library of Congress Control Number (LCCN). This number applies to the bibliographic record created by the Library of Congress and used by libraries. This is a great market to approach so I highly recommend it. The LCCN should be placed on the copyright page of your book.
This number cannot be issued to a book that has already been published so be sure to obtain this number well in advance of any publication date. It is also only available to U.S. Publishers that provide a place of publication on the copyright page and maintain an office in the U.S. that is capable of being contacted and answering questions.
You can find more information at http://pcn.loc.gov





