The Concise Oxford Dictionary of Opera (Oxford Paperback Reference)

This greatly enlarged edition includes many rewritten and updated entries as well as numerous new articles on composers and performers, details of casts at first performances, and much additional information on the development of opera in different countries. Comprehensive, compact, and inexpensive, this is an indispensable reference work for all opera enthusiasts.

This is a handy, portable guide to opera: composers, artists, national trends. The authors have done a good job of ferreting out information and putting it together. If you just need to check which opera has a prima donna called Isabella (Rossini's L'Italiana in Algeri) or the definition of a covered tone, this is a good book to grab.