An author, upon reading my approach to indexing, expressed concern that sources would not appear in the index. Here is my reply:
Sources are usually not included in the index—they are already in the notes and bibliography, and sometimes in the text as well. If you write two paragraphs about the theories of Author X or mention Author X throughout the book, the indexer will most likely include Author X in the index (I would). But if you mention in passing that “According to Author Z, aardvarks prefer termites to ants,” and the only other time Author Z shows up is in the notes and bibliography, then Author Z would not appear in the draft index that I would send to you. Once you have that draft, if you have strong feelings about including Author Z, there would be no problem with adding one or two entries. You don’t want the index to repeat the bibliography and notes, nor do you want the index to be inundated with names of other researchers whose work might only be tangential to your book.
Aside: your indexer will certainly index any substantive notes—just not the ones that are nothing but a citation.