Dick Eastman has put up an extremely helpful post on using WorldCat.org, and people who’ve attended one of my classes know what a big WorldCat.org booster I am:
“WorldCat is the world's largest network of library content and services. It is an online library catalog that lets you look up items in libraries around the world. The items available include books, electronic documents, journals, microform, and audio and video recordings.
Best of all, WorldCat is available to everyone free of charge. WorldCat libraries provide access to their catalogs on the Web, where most people start their search for information. By using the WorldCat.org catalog, you can search the collections of libraries in your community and thousands more around the world.
Please keep in mind that these are catalogs of available books, not the text of the books. WorldCat is useful in at least three situations:
1. You know which item you want, but you don't know which libraries have it
2. You want to know what items exist on a particular subject (or by a particular author)
3. You need more information about an item (for instance, to get information for a proper citation in your bibliography)
WorldCat identifies the book's title, author, publisher, and more and then shows the participating libraries where the book is available. “