The Library of Congress is the official repository for all the books written in the United States. It assigns a unique number to each new book for use in its catalog.
Before a book is published, the publisher informs the Library of Congress of the new book and sends information to help librarians decide where it will fit on their shelves. The first step is to apply to the Preassigned Control Number Program online to get your LCCN (www.pcn.loc.gov) It is emailed to you in 24 hours. You cannot get an LCCN once your book is published.