The Library of Congress Control Number (LCCN) is a serially based system of numbering cataloging records in the Library of Congress in the United States. It has nothing to do with the contents of any book, and should not be confused with Library of Congress Classification. 

The LCCN numbering system has been in use since 1898, at which time the acronym LCCN originally stood for Library of Congress Card Number. It has also been called the Library of Congress Catalog Card Number, among other names. The Library of Congress prepared cards of bibliographic information for their library catalog and would sell duplicate sets of the cards to other libraries for use in their catalogs. This is known as centralized cataloging. Each set of cards was given a serial number to help identify it. 

Although most of the bibliographic information is now electronically created, stored, and shared with other libraries, there is still a need to identify each unique record, and the LCCN continues to perform that function. 

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 reach 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.
Does anyone outside of the library see this information?
which the Library of Congress and third parties make available on the Web and through other media