It’s a feature to make it easier for you to find books in libraries around the world. When you search for a book or a topic using Google Book Search, you’re searching both books we’ve scanned for the index and basic information (or “metadata”) from library catalogs.
The way it works is simple. If your search query suggests you’re looking for a particular book – for example, if you enter the title of a book – you’ll see search results from books we’ve digitized, plus, at the bottom of the search results page, relevant library catalog results to help you find books in nearby libraries.
You can also choose the “Library catalogs” search option on the Advanced Search page. Bear in mind that library catalogs contain only metadata – basic information like author, title and subject – so results from catalogs are not based on the full text of books.