What is the Latin word for 'librarian'?

There are many forms:

1. liber, -era, -erum: free (social or politically), as a noun: a free man (or woman), etc

2. liber, -eri: son, child

3. Liber, -eri: Baccus (the god) or wine

4. liber, -bri: book, letter or any sort of text.