The Collaborative International Dictionary of English v.0.48:

Librettist \Li*bret"tist\ (l[i^]*br[e^]t"t[i^]st), n.
   One who makes a libretto.
   [1913 Webster]

WordNet (r) 3.0 (2006):

librettist
    n 1: author of words to be set to music in an opera or operetta