The Library of Congress Control Number (LCCN) is the number associated with the bibliographic record created by the Library of Congress or another library for a given book. Strictly speaking the LCCN is the control number for the bibliographic record, not the book
A Preassigned Control Number (PCN) is a Library of Congress Control Number which has been "preassigned" to a given work prior to the work's publication. A publisher of an eligible work may obtain a PCN if the work has not yet been published. Published works are otherwise assigned a LCCN during the cataloging process, if they have been selected for addition to the Library of Congress' collections.