1) Author 2) Writer 3) Writer in a special field 
• (n.) One who makes a libretto.
Li·bret'tist (lĭ*brĕt'tĭst) noun One who makes a libretto. 
[n] - author of words to be set to music in an opera or operetta
librettist 1. An author of words to be set to music in an opera or operetta. 2. A writer of the words for a dramatic musical work; such as, an opera or musical.
The person who writes the libretto.
noun author of words to be set to music in an opera or operetta
The person who writes the libretto, often a poet or playwright.