A national library is generally the official library of a country, maintained by that nation's resources. National libraries usually try to collect all publications issued in their countries. (Often they are automatically sent a copy or two of any book published/copyrighted in that country.) Hence the catalog of a national library is often the best guide to publications issued in the country in question. The Library of Congress and the British Library are well-known national libraries, and
http://www.library.uq.edu.au/natlibs/ lists many more which have web pages and online catalogs.