If it is a school library, I think it is more appropriate to call a teacher! The school is a place to educate people, and it is called the children of scholars! Address, its meaning is also respect for each other! If it's an off-campus library, it's the simplest "hello", and neither you nor the other party will feel uncomfortable ... (That's what I almost call each other when I go to the bookstore to read) Hehe.