The Library of Congress Control Number (LCCN) is a serially-based system of numbering and cataloging records in the United States Library of Congress.
In its most basic form, the number includes the year of registration and a serial number. The year has two digits for 1898 to 2000, and four digits beginning in 2001. The three ambiguous years (1998-2000) are distinguished by the size of the serial number.
Librarians all over the world use this unique identifier in the process of cataloging most books which have been published in the United States. It helps them access the correct cataloging data (known as a cataloging record), which the Library of Congress and third parties make available on the Web and through other media.
The Library of Congress reserves the right to refuse cataloguing titles. They do not provide catalog numbers for books under 50 pages unless they are a children's book. They also do not catalog transitory or consumable materials, such as coloring books, comic books, and cut-out books. The Library of Congress will not assign card catalog numbers for books that fall under any of these categories.
Note: If this service is purchased after your book has been published, additional fees may apply.