WorldCat is a catalog that includes records from thousands of libraries around the world. This makes it a great place to go when you are doing in-depth research and are really trying to find every book on a topic. Free access to WorldCat is provided through their web interface Worldcat.org.
The video below shows you:
WorldCat is a great resources, but its size and the number of people who import records into it can make it very messy and hard to search. I think this has gotten significantly more problematic over the last few years. If you are struggling with WorldCat, here are some other options.