Librarians have many specialized tools to help them do their work, including a single catalog of library collections all over the world. You can use that one, too!
WorldCat aspires to be a universal library catalog. It hasn’t made it yet, but it shows the holdings of about 27,000 different libraries in more than 170 different countries. Even though it does not contain the holdings of every library in the world, it easily lives up to its slogan “the world’s largest library catalog.”