Bel Canto des Wortes
A truly wonderful, insightful, and provocative book from a student of Francesco Lamperti, Cornelie van Zanten's Bel Canto des Wortes (1911) should be required reading in vocal pedagogy programs.
Serious, real-deal, historically-informed old Italian school of singing, it puts to shame recent volumes that claim as much but offer far less.
Use a Google translation app to get your feet wet. Then swim.
Find Bel Canto des Wortes on VOICETALK's download page.