The National Union Catalog of pre-1956 Imprints is a cumulative author list representing Library of Congress printed cards and titles reported by other American libraries. It was compiled and edited with the cooperation of the Library of Congress and the National Union Catalog Subcommittee of the Resources Committee of the Resources and Technical Services Division, American Library Association, published 1968-1981. It was often used as a source for author attributions, but much scholarship has superseded the information found in it. It has been largely eclipsed by WorldCat and the Internet.

Available through HathiTrust: 

National Union Catalog, pre-1956 Imprints

see also: abbreviations found in cataloging records, authority file, general note(s), OCLC / WorldCat