Not sure about your uni - at leeds, they actualy do have a massive database - Its got not only books but every essay the uni and other uni's has been given, so they can check it globaly
For example, when I gave in a essay last term, I gave it online and was actually shown the database - it highlights in red anything thats word for word something in the database - one of my sentences perfectly matched one from an essay apparently from the university of California, if that helps give a sense of it.
So yeah, even if your uni hasn't copied every book into it, other unis have put alot of theirs, so the number of books is presumably utterly massive