WorldCat is the world's largest network of library content and services. It allows you to search for particular items in the collections of libraries around the world. This is especially helpful to genealogists as we can learn what publications are available and where.
For example, if you are looking for a particular book on Union County, Kentucky or even want to see all books available on the county, go to WorldCat and type in the name of the book or just Union County. Available books will be listed and at which libraries. If the library is nearby, it is simple enough to visit, but if the library is at a distance, ask your local library if the book can be interlibrary loaned to your local library.
I just acquired through WorldCat and interlibrary loan a book that I was sure would never be available to me. It was loaned by the Illinois State Library and I can use it for three weeks. The only cost to me was for mailing. This convenience in borrowing these materials somehow makes the world seem just a little smaller.