The card catalog. Tiny wooden drawers filled with index cards, organized by a numbering system that made perfect sense to librarians and no sense to anyone else.
You had to learn it in school. They made you practice. 'Find a book about volcanoes.' You would thumb through the cards, write down a call number like 551.21, and then wander the stacks looking for the right shelf. It was a treasure hunt every time.
The library was quiet in a way that nothing is quiet anymore. Real quiet. The kind of quiet where you could hear someone turn a page from across the room. And the smell. Old paper and wood polish. I miss that smell more than almost anything from childhood.