A good index is like a smart mirror. It reflects the text while also processing it, mediating between the content and the reader. Authors will sometimes write their own indexes, encouraged by the fact that most book contracts give them the responsibility for getting it done. But I encourage seeking outside help for this task. An indexer is trained to have what might be called an invested objectivity, a professional empathy for the work and its language. A confused index is a barrier between the book and its audience; a good one is an invitation.
A downloadable sample index I wrote for José Ortega y Gasset, The Dehumanization of Art and Other Essays on Art, Culture, and Literature (Princeton University Press, 1968), is available below.