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

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "librettist":
   Meistersinger, Parnassian, arch-poet, arranger, ballad maker,
   balladeer, balladist, balladmonger, bard, beat poet, bucoliast,
   choreographer, comedian, composer, contrapuntist, dramatist,
   dramatizer, dramaturge, elegist, epic poet, ethnomusicologist,
   farcer, farceur, farceuse, farcist, fili, gagman, harmonist,
   harmonizer, hymnist, hymnographer, hymnologist, idyllist, imagist,
   joke writer, jokesmith, jongleur, laureate, madrigalist,
   major poet, maker, melodist, melodizer, melodramatist, mimographer,
   minnesinger, minor poet, minstrel, modernist, monodramatist,
   musicographer, musicologist, occasional poet, odist, orchestrator,
   pastoral poet, pastoralist, play doctor, play fixer, playwright,
   playwriter, poet, poet laureate, poetress, rhapsode, rhapsodist,
   satirist, scenario writer, scenarioist, scenarist, scop, scorer,
   scriptwriter, skald, song writer, songsmith, sonneteer, symbolist,
   symphonist, tone poet, tragedian, troubadour, trouveur, trovatore,
   tunesmith, vers libriste, vers-librist