There are a number of non-commercial meta-catalogs that cover the stock of various (often national) libraries. For example, Germany, Austria and Switzerland have a common cross-library interlibrary loan catalogue called "Karlsruher Virtueller Katalog". In the United States, there is the Catalogue of the Library of Congress, which probably has the largest coverage in the country. Italy has the "OPAC SBN" of the "Istituto Centrale per il Catalogo Unico".
In terms of coverage, meta cataloges like these represent the submerged part of the iceberg that Google Books only sees the tip of. However, only small parts, if any, of the inventory are accessible for full-text search.
The use of these catalogs is free of charge, but small fees are charged for checking out books or for ordering copies of book sections, journal articles, or other documents.
In general, it's always a good idea to talk to a librarian for particular queries or about search strategies.