libretto

Description: 

The words or text to which a cantata, oratorio, opera, operetta, or other work for the musical stage is set, often published in the form of a small booklet for enthusiasts who wish to follow along while attending a performance or listening to a sound recording. The person who writes a libretto is the librettist, for example, Edna St. Vincent Millay in the case of The King's Henchman, an opera composed by Deems Taylor.

Source: 
Reitz Joan M. (2004)