David Hamilton & William E. Studwell 
Opera Plot Index [PDF ebook] 

First Published in 1990. Information about individual operas and other types of musical theater is scattered throughout the enormous literature of music. This book is an effort to bring that data together by comprehensively indexing plots and descriptions of individual operatic background, criticism and analysis, musical themes and bibliographical references. The principal audience for this general reference guide will be for the non-specialist, but its hoped that persons specialising in opera would also find it useful.