Google Book Search is a good way to find books and then buy them or borrow them from a library. Now the site includes the option to search library catalogs: you can either use the advanced search or look at the bottom of the search results. Google says there is a pretty good chance that libraries are close to you, but that doesn't happen all the time, as Google has information only about libraries from 30 countries.
I think it would've been much better to show the libraries where a book is available on the book page, right next to the bookstores where you can buy it.
Google's Digital Library of Alexandria
How to use Google and become a better student