An "index" is a more detailed alphabetized list of terms with page numbers taking you to the particular song(s) in which they occur. Ideally they are generated by the publisher or editor of a volume and added at the back. Should one be compiled, it would be handy if saved in a PDF with links to the pages if the book is published anywhere. Contents are different, and list of first lines is different, though all three are useful in this kind of book.