I worked part time in a departmental library while a full time university student. It's not volunteering, but then (like a volunteer) I didn't have any training in library science.
Mostly what they had me do was physically very strenuous. Think of moving an entire stack of heavy bound journals and books across the room. Even with a cart, those books are heavy to lift up to the top shelf and elsewhere, and the lifting never ends. And then, put away the returned books. Then when they don't fit, move books from shelf to shelf so that the shelves all have a little leeway.
At the end of the day, I was sore and exhausted. Working in a library SOUNDS fun, intellectual, and interesting, but for me the reality of it was somewhat similar to what I imagine digging ditches might be like.
I also got to stamp books with the date when they were checked out, and type out cards for the card catalog (I don't suppose those are used any more), compile circulation statistics, and search for books that people were looking for, and things like that. But mostly, I just lugged books around.
I may volunteer after ER, but not at a library.