I have found this manual to be quite good, which is convenient since there is practically nothing else out there for Spanish singing that I know of. Castel's book provides quick-reference as well as detailed guides to pronouncing Spanish, and he gives good coverage of Spanish variants that one may come across. The repertoire suggestions are handy as well. About the only thing I can think of to improve the edition would be a CD with audio samples of the various sounds, since there are some that are indigenous to Castilian Spanish. This of course would drive up the price, but I think it'd be worth it since most people purchasing this book *genuinely* want to sing well in Spanish.