From my experience (in Poland), university libraries are open for whoever wants to use them. My department does not even care whether people who attend classes

From my experience (in Poland), university libraries are open for whoever wants to use them. My department does not even care whether people who attend classes are actually students of my department -- as long as there are free chairs and desks, you're welcome to attend almost any course. If you talk about it with dean and professor responsible for the course, you can even pass it this way, so that if you actually enroll in some degree program in future, you don't need to do it again. I actually have friends who passed courses on my university when they were in high school.