The NUC of Pre-1956 Imprints was an important resource for verifying bibliographic information and finding copies of books before the advent of large electronic bibliographic databases, such as WorldCat; the massive size and weight of the set make it less useful now. However, given that approximately 27% of the books listed in the NUC Pre-1956 Imprints were not in listed WorldCat as of 2005, it remains an extremely valuable tool for researchers.
The Library of Congress began its union catalog project in 1901 in an attempt to locate and note the location of a copy of every important book in the United States. With financial assistance from John D. Rockefeller Jr., the collection grew to over 11 million cards. Copies of these cards were distributed to a number of libraries around the country. Eventually the cards for all materials catalogued by the cooperating libraries were reproduced and issued serially in printed volumes as the National Union Catalog, supplementing the Library of Congress Catalog of Printed Books. Monthly NUC catalogs were cumulated quarterly, annually, and multi-annually.
In an effort to simplify research, it was decided in the 1960s to collect and publish all of the references to pre-1956 imprints in a single alphabetical listing. Mansell Information/Publishing Ltd., the company which created the British Museum library catalog, won the contract to publish the proposed union catalog. Over the next 14 years, about five 600-page volumes were published each month until the NUC of Pre-1956 Imprints was completed, largely superseding the older Library of Congress Catalog of Printed Books.