A librarian is a person who works in a library, organizing and managing of information services or materials for those with information needs.
Mr. Atoz oversaw a library on the planet Sarpeidon. He had replicas of himself to help him in the library. (TOS: "All Our Yesterdays")
Mira Romaine, a lieutenant on her first space assignment, was heading for Memory Alpha, where she was to be the librarian. The planetoid had been designed as an open research facility containing records of all human knowledge. On the way, a community of alien beings' thoughts tried to take over Mira's body, but she was able to resist. Although the beings had killed everyone on Memory Alpha and caused catastrophic damage to the databases, Mira went back there to try to rebuild it. Scotty was in love with her. (((TOS|The Lights of Zetar)))
After being captured on Delta Theta III, James T. Kirk remarked that "There are times, Mr. Spock, when I think I should have been a librarian." Spock observed that "The job of librarian would be no less challenging, Captain, but it would undoubtedly be a lot less dangerous." (TAS: "Bem")
Appendices
A librarian in a deleted scene of Star Trek: Insurrection
Background Information
In a deleted scene in Star Trek: Insurrection, a librarian serving on the USS Enterprise-E appeared (played by Lee Arnone-Briggs). She was a sciences division lieutenant.