Sites like Google Books and the Open
Library (which aims to have a web
page for every book ever published) have many Public Domain available
online in full text.
Open Library's
listing of books on vocal pedagogy
Google
Books listing of books on vocal pedagogy
For books not available in full text, a convenient link leads to sites where the book can be purchased. Check RuCat for library availablity or consider using Interlibrary Loan.