THE SPLENDID ART OF OPERA by Ethan Mordden; Methuen; 413 pages; $19.95
THE GOLDEN CENTURY OF ITALIAN OPERA by William Weaver Thames & Hudson; 256 pages; $27.50
The rich Florentines who crowded into the house of Jacopo Corsi in 1597 were unaware that they were about to witness a miracle. Dafne, the work they were to applaud, was an unprecedented marriage of music and drama: the world's first opera. Jacopo Peri's score is lost, but it must have been a success, because soon nearly every composer in Italy was trying le nuove musiche.