Ha! Finally caught up. I must profess my love for WorldCat — I use it all the time when helping patrons at the Internet Public Library because it’s the easiest way for me to tell what libraries are closest to them (if they’ve given me their location). It’s easy enough to show folks how to look up a book on this service and locate the closest copy, even if they haven’t given me their location.
If you find a book on Google Books, you can click through from there (use the “Find in a library” link) to see the book’s WorldCat results including which libraries near you may have it. Often when I stumble upon a book this way, I check out the subject headings for the book (see the red box below; click on the image for a larger version):
If you’re really lucky, the subject headings for your book will include surnames — click on the related headings for more titles focused on these subjects. It’s a serendipitous way to discover new resources.