You can find librettos and synopses in the Fine Arts Library's collections, eBooks, and on dozens of web sites. Below are just a few to get you started. Another great source for librettos is booklets included with CDs of operas in the Fine Arts Library.
The University of Kentucky Libraries uses the Library of Congress Classification System for call numbers. Librettos can be found under ML50 and then alphabetical by composer. Collections of librettos can be found under ML48 and ML49. Check the Reference section, too, for librettos with IPA. You can browse the shelves or search by call number in InfoKat Discovery.
Browse librettos by call number (ML48-ML50) in InfoKat Discovery.
The Little Fine Arts Library Reference section holds Leyerle's publications of librettos, which are sometimes referred to as the "Castel IPA books." Below are a few of the titles in this series:
From the Music History Department of the German Historical Institute in Rome, this collection contains 1,500 historical libretto prints from L’Andromeda by Francesco Manelli (1637) and the early days of opera in Venice, since the opening of the first public opera houses, up to the year 1734.