Sonneck, Oscar George Theodore and Albert Schatz. Catalogue of opera librettos printed before 1800. Washington, D.C.: Govt. Printing Office, 1914.
A comprehensive, representative collection of opera libretti of the Library of Congress. Over 12,240 libretti of which 12,000 are from operas with the remaining libretti coming from oratorios and cantatas.
Collection from Albert Schaatz. These catalogs were passed to the Library of Congress by the author when in the last days of his life. Includes operas, mainly in Italian, French, and German, printed before 1800.
It has two volumes. In each volume, the end of each entry has a call number. Most of them are Schaatz and Library of Congress (ML). The first volume is alphabetized by title and the second has three subdivisions. The first is names of librettist alphabetized by last name. Each entry includes the titles of all libretti they wrote. Some of these entries also have location and year. The second is names of composers and also includes works and dates. The third subdivision is aria titles. Each entry contains title of opera, name of composer, author, place, theatre, incipits, dates of first performance and of the first repetition in other cities. Names of composers or authors are never given in completion. For example instead of Maria Luigi Zenobio arlo Salvatore Cherubini only Cherubini, assuming the reader is familiar with these names already.
Catalog is very detailed and easy to understand. Great source if looking for materials at the Library of Congress. Includes nicknames of people.