One who keeps a collection so others can find what they need in it. The librarian's work is retrieval, not ownership.
Latin liber (book; originally the inner bark of a tree, written on before papyrus reached Rome), with an agent suffix; one who keeps a collection so others can find what they need in it, without owning it.
The work here is retrieval, never ownership — a collection kept so that whoever needs something in it can actually find it.