Hi @BryanBurkhead,
What you see in the +People dropdown is what Canvas calls roles. Sometimes a role might match someone's title, but that's not always going to be the case unless a lot of custom roles are created (which is doable, but can create a permissions nightmare for admins). Regardless of a person's title, their role in Canvas indicates what they can do in Canvas. So in this case, librarians are teaching the course in some manner.
As for you not seeing the teacher role when you click the +People button, that's likely something your Canvas team is controlling via permissions. Many times schools/institutions will allow teachers to add assistants or something on their own, but things like a teacher or student would only be added by an integration with your school/institution student information system. You could reach out to your Canvas team to find out more about the processes in place for you.
I hope this helps a bit!
-Chris